[LangRef] Update description of Constant Expression

[bjope]

Summary

When describing trunc/zext/sext/ptrtoint/inttoptr in the chapter
about Constant Expressions we now simply refer to the Instruction
Reference. As far as I know there are no difference when it comes
to the semantics and the argument constraints. The only difference
is that the syntax is slighly different for the constant expressions,
regarding the use of parenthesis in constant expressions.
Referring to the Instruction Reference is the same solution as
already used for several other operations, such as bitcast.

The main goal was to add information that vector types are allowed
also in trunc/zext/sext/ptrtoint/inttoptr constant expressions.
That was not explicitly mentioned earlier, and resulted in some
questions in the review of https://reviews.llvm.org/D38546

[efriedma]

LGTM.

docs/LangRef.rst

 
 Constant expressions are used to allow expressions involving other
 constants to be used as constants. Constant expressions may be of any
 :ref:`first class <t_firstclass>` type and may involve any LLVM operation
 that does not have side effects (e.g. load and call are not supported).
 The following is the syntax for constant expressions:
 
 ``trunc (CST to TYPE)``
-    Truncate a constant to another type. The bit size of CST must be
-    larger than the bit size of TYPE. Both types must be integers.
+    Perform the :ref:`trunc operation <i_trunc>` on constants.
 ``zext (CST to TYPE)``
-    Zero extend a constant to another type. The bit size of CST must be
-    smaller than the bit size of TYPE. Both types must be integers.
+    Perform the :ref:`zext operation <i_zext>` on constants.
 ``sext (CST to TYPE)``
-    Sign extend a constant to another type. The bit size of CST must be
-    smaller than the bit size of TYPE. Both types must be integers.
+    Perform the :ref:`sext operation <i_sext>` on constants.
 ``fptrunc (CST to TYPE)``
     Truncate a floating point constant to another floating point type.
     The size of CST must be larger than the size of TYPE. Both types
     must be floating point.
 ``fpext (CST to TYPE)``
     Floating point extend a constant to another type. The size of CST
     must be smaller or equal to the size of TYPE. Both types must be
     floating point.
``uitofp (CST to TYPE)``
     won't fit in the floating point type, the results are undefined.
 ``sitofp (CST to TYPE)``
     Convert a signed integer constant to the corresponding floating
     point constant. TYPE must be a scalar or vector floating point type.
     CST must be of scalar or vector integer type. Both CST and TYPE must
     be scalars, or vectors of the same number of elements. If the value
     won't fit in the floating point type, the results are undefined.
 ``ptrtoint (CST to TYPE)``
-    Convert a pointer typed constant to the corresponding integer
-    constant. ``TYPE`` must be an integer type. ``CST`` must be of
-    pointer type. The ``CST`` value is zero extended, truncated, or
-    unchanged to make it fit in ``TYPE``.
+    Perform the :ref:`ptrtoint operation <i_ptrtoint>` on constants.
 ``inttoptr (CST to TYPE)``
-    Convert an integer constant to a pointer constant. TYPE must be a
-    pointer type. CST must be of integer type. The CST value is zero
-    extended, truncated, or unchanged to make it fit in a pointer size.
+    Perform the :ref:`inttoptr operation <i_inttoptr>` on constants.
     This one is *really* dangerous!
 ``bitcast (CST to TYPE)``
-    Convert a constant, CST, to another TYPE. The constraints of the
-    operands are the same as those for the :ref:`bitcast
-    instruction <i_bitcast>`.
+    Convert a constant, CST, to another TYPE.
+    The constraints of the operands are the same as those for the
+    :ref:`bitcast instruction <i_bitcast>`.
 ``addrspacecast (CST to TYPE)``
     Convert a constant pointer or constant vector of pointer, CST, to another
     TYPE in a different address space. The constraints of the operands are the
     same as those for the :ref:`addrspacecast instruction <i_addrspacecast>`.
 ``getelementptr (TY, CSTPTR, IDX0, IDX1, ...)``, ``getelementptr inbounds (TY, CSTPTR, IDX0, IDX1, ...)``
     Perform the :ref:`getelementptr operation <i_getelementptr>` on
     constants. As with the :ref:`getelementptr <i_getelementptr>`
     instruction, the index list may have one or more indexes, which are
     required to make sense for the type of "pointer to TY".
 ``select (COND, VAL1, VAL2)``
     Perform the :ref:`select operation <i_select>` on constants.
 ``icmp COND (VAL1, VAL2)``
-    Performs the :ref:`icmp operation <i_icmp>` on constants.
+    Perform the :ref:`icmp operation <i_icmp>` on constants.
 ``fcmp COND (VAL1, VAL2)``
-    Performs the :ref:`fcmp operation <i_fcmp>` on constants.
+    Perform the :ref:`fcmp operation <i_fcmp>` on constants.
 ``extractelement (VAL, IDX)``
     Perform the :ref:`extractelement operation <i_extractelement>` on
     constants.
 ``insertelement (VAL, ELT, IDX)``
     Perform the :ref:`insertelement operation <i_insertelement>` on
     constants.
 ``shufflevector (VEC1, VEC2, IDXMASK)``
     Perform the :ref:`shufflevector operation <i_shufflevector>` on

 
 Conversion Operations
 ---------------------
 
 The instructions in this category are the conversion instructions
 (casting) which all take a single operand and a type. They perform
 various bit conversions on the operand.
 
+.. _i_trunc:
+
 '``trunc .. to``' Instruction
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 """""""
 
 ::
 

 
 .. code-block:: llvm
 
       %X = trunc i32 257 to i8                        ; yields i8:1
       %Y = trunc i32 123 to i1                        ; yields i1:true
       %Z = trunc i32 122 to i1                        ; yields i1:false
       %W = trunc <2 x i16> <i16 8, i16 7> to <2 x i8> ; yields <i8 8, i8 7>
 
+.. _i_zext:
+
 '``zext .. to``' Instruction
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 """""""
 
 ::
 

 """"""""
 
 .. code-block:: llvm
 
       %X = zext i32 257 to i64              ; yields i64:257
       %Y = zext i1 true to i32              ; yields i32:1
       %Z = zext <2 x i16> <i16 8, i16 7> to <2 x i32> ; yields <i32 8, i32 7>
 
+.. _i_sext:
+
 '``sext .. to``' Instruction
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Syntax:
 """""""
 
 ::