Update profile during memory instrinsic optimization

Summary:
Ensure that the new merge BB (which contains the rest of the original BB
after the mem op being optimized) gets a profile frequency, in case
there are additional mem ops later in the BB. Otherwise they get skipped
as the merge BB looks cold.

Reviewers: davidxl, xur

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32447

llvm-svn: 301244