[AliasSet] Teach the alias set how to handle atomic memcpy/memmove/memset

[AliasSet] Teach the alias set how to handle atomic memcpy/memmove/memset

Summary:
The atomic variants of the memcpy/memmove/memset intrinsics can be treated
the same was as the regular forms, with respect to aliasing. Update the
AliasSetTracker to treat the atomic forms the same was as the regular forms.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333551 91177308-0d34-0410-b5e6-96231b3b80d8