[PGO] Guard the memcmp/bcmp size value profiling instrumentation behind flag.

Summary:
Follow up D79751 and put the instrumentation / value collection side (in
addition to the optimization side) behind the flag as well.

Reviewers: davidxl

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80646