[PGO] Update VP metadata after memory intrinsic optimization

Summary:
Leave an updated VP metadata on the fallback memcpy intrinsic after
specialization. This can be used for later possible expansion based on
the average of the remaining values.

Reviewers: davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34164

llvm-svn: 305321