[InstCombine] Disable strcmp->memcmp transform for MSan.

Summary:
The strcmp->memcmp transform can make the resulting memcmp read
uninitialized data, which MSan doesn't like.

Resolves https://github.com/google/sanitizers/issues/993.

Reviewers: eugenis, xbolva00

Reviewed By: eugenis

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D52272

llvm-svn: 342582