[Analysis] RemoveTotalMemInst counting in InstCount to avoid reading back other Statistic variables

Summary:
Previously, we counted TotalMemInst by reading certain instruction counters before and after calling visit and then finding the difference. But that wouldn't be thread safe if this same pass was being ran on multiple threads.

This list of "memory instructions" doesn't make sense to me as it includes call/invoke and is missing atomics.

This patch removes the counter all together.

Reviewers: hfinkel, chandlerc, davide

Reviewed By: davide

Subscribers: davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D33608

llvm-svn: 308260