[MemorySSA] Update MSSA for non-conventional AA.

[MemorySSA] Update MSSA for non-conventional AA.

Summary:
Regularly when moving an instruction that may not read or write memory,
the instruction is not modelled in MSSA, so not action is necessary.
For a non-conventional AA pipeline, MSSA needs to explicitly check when
creating accesses, so as to not model instructions that may not read and
write memory.

Reviewers: george.burgess.iv

Subscribers: Prazek, sanjoy.google, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67562

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372137 91177308-0d34-0410-b5e6-96231b3b80d8