[PGO] Minor cleanup for count instruction in SelectInstVisitor.

Summary:
NSIs can be double-counted by different operations in
SelectInstVisitor. Sink the the update to VM_counting mode only.
Also reset the value for each counting operation.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: xur, llvm-commits

Differential Revision: https://reviews.llvm.org/D30999

llvm-svn: 297892