Do not want to use BFI to get profile count for sample pgo

Summary: For SamplePGO, we already record the callsite count in the call instruction itself. So we do not want to use BFI to get profile count as it is less accurate.

Reviewers: tejohnson, davidxl, eraman

Reviewed By: eraman

Subscribers: sanjoy, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D36025

llvm-svn: 309964