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]

ping
Adding Phillip because he might be interested in the topic

[sanjoy]

Minor comments inline.

docs/AliasAnalysis.rst

139

A better wording would be "A `MustAlias` response does not imply that the pointers compare equal."

docs/LangRef.rst

5121

This is minor, but I'd s/Because/Since/

Even better, I'd put the (IMO more important) warning before its justification:

The invariant.group metadata must be dropped when replacing one pointer by another based on aliasing information. This is because invariant.group is tied to the SSA value of the pointer operand.

5129

Not sure what this line adds -- can you give an example here as well?

[Prazek]

Thanks Sanjoy!

docs/LangRef.rst

5129

I guess I will just delete that because it doesn't add any new info

[Prazek]

Sanjoy comments

[sanjoy]

lgtm!

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 does not imply 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"}
 
+The invariant.group metadata must be dropped when replacing one pointer by
+another based on aliasing information. This is because invariant.group is tied
+to the SSA value of the pointer operand.
+
+.. 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
+
+
 '``type``' Metadata
 ^^^^^^^^^^^^^^^^^^^
 
 See :doc:`TypeMetadata`.
 
 '``associated``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^