Expose isLegalToPromot as a global helper function so that SamplePGO pass can...

Expose isLegalToPromot as a global helper function so that SamplePGO pass can call it for legality check.

Summary: SamplePGO needs to check if it is legal to promote a target before it actually promotes it.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29306

llvm-svn: 293559