Invariant.group and mustalias docs fixes

[Prazek]

Summary

Alias analysis would like to know that
invariant.group.barrier returns pointer that mustalias,
but this can't imply that we can replace one pointer with another

[Prazek]

Reposting again because of forgotten llvm-commits

docs/AliasAnalysis.rst

 The ``MayAlias`` response is used whenever the two pointers might refer to the
 same object.
 
 The ``PartialAlias`` response is used when the two memory objects are known to
 be overlapping in some way, but do not start at the same address.
 
 The ``MustAlias`` response may only be returned if the two memory objects are
 guaranteed to always start at exactly the same location. A ``MustAlias``
-response implies that the pointers compare equal.
+response not implies that the pointers compare equal.
 
 The ``getModRefInfo`` methods
 -----------------------------
 
 The ``getModRefInfo`` methods return information about whether the execution of
 an instruction can read or modify a memory location.  Mod/Ref information is
 always conservative: if an instruction **might** read or write a location,
 ``ModRef`` is returned.

docs/LangRef.rst

.. code-block:: llvm
    ...
    declare void @foo(i8*)
    declare i8* @getPointer(i8*)
    declare i8* @llvm.invariant.group.barrier(i8*)
    
    !0 = !{!"magic ptr"}
    !1 = !{!"other ptr"}
 
+Because `invariant.group` is tied to the SSA value of the pointer operand,
+replacing one pointer by another based on `mustalias` returned from Alias
+Analysis can't be done without discarding the `invariant.group` metadata.
+.. code-block:: llvm
+  %v = load i8, i8* %x, !invariant.group !0
+  ; if %x mustalias %y then we can replace the above instruction with
+  %v = load i8, i8* %y
+
+Where replacing by SSA values equivalence is not required to discard
+`invariant.group` metadata.
+
+
+
 '``type``' Metadata
 ^^^^^^^^^^^^^^^^^^^
 
 See :doc:`TypeMetadata`.
 
 '``associated``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^