[LangRef] fptosi and fptoui return poison on overflow.

[efriedma]

Summary

I think we assume poison, not undef, for certain transforms we currently do. In any case, we should clarify the language here.

[nlopes]

LGTM.

The C++ standard defines this as UB: http://eel.is/c++draft/conv.fpint#1
"A prvalue of a floating-point type can be converted to a prvalue of an integer type. The conversion truncates; that is, the fractional part is discarded.
The behavior is undefined if the truncated value cannot be represented in the destination type."

docs/LangRef.rst

``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.
 ``fptoui (CST to TYPE)``
     Convert a floating-point constant to the corresponding unsigned
     integer constant. TYPE must be a scalar or vector integer type. CST
     must be of scalar or vector floating-point type. Both CST and TYPE
     must be scalars, or vectors of the same number of elements. If the
-    value won't fit in the integer type, the results are undefined.
+    value won't fit in the integer type, the result is a
+    :ref:`poison value <poisonvalues>`.
 ``fptosi (CST to TYPE)``
     Convert a floating-point constant to the corresponding signed
     integer constant. TYPE must be a scalar or vector integer type. CST
     must be of scalar or vector floating-point type. Both CST and TYPE
     must be scalars, or vectors of the same number of elements. If the
-    value won't fit in the integer type, the results are undefined.
+    value won't fit in the integer type, the result is a
+    :ref:`poison value <poisonvalues>`.
 ``uitofp (CST to TYPE)``
     Convert an unsigned 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.
 ``sitofp (CST to TYPE)``
     Convert a signed integer constant to the corresponding floating-point

 ``ty`` is a vector floating-point type, ``ty2`` must be a vector integer
 type with the same number of elements as ``ty``
 
 Semantics:
 """"""""""
 
 The '``fptoui``' instruction converts its :ref:`floating-point
 <t_floating>` operand into the nearest (rounding towards zero)
-unsigned integer value. If the value cannot fit in ``ty2``, the results
-are undefined.
+unsigned integer value. If the value cannot fit in ``ty2``, the result
+is a :ref:`poison value <poisonvalues>`.
 
 Example:
 """"""""
 
 .. code-block:: llvm
 
       %X = fptoui double 123.0 to i32      ; yields i32:123
       %Y = fptoui float 1.0E+300 to i1     ; yields undefined:1

 ``ty`` is a vector floating-point type, ``ty2`` must be a vector integer
 type with the same number of elements as ``ty``
 
 Semantics:
 """"""""""
 
 The '``fptosi``' instruction converts its :ref:`floating-point
 <t_floating>` operand into the nearest (rounding towards zero)
-signed integer value. If the value cannot fit in ``ty2``, the results
-are undefined.
+signed integer value. If the value cannot fit in ``ty2``, the result
+is a :ref:`poison value <poisonvalues>`.
 
 Example:
 """"""""
 
 .. code-block:: llvm
 
       %X = fptosi double -123.0 to i32      ; yields i32:-123
       %Y = fptosi float 1.0E-247 to i1      ; yields undefined:1