[PGO][PGSO] Use IsColdXNthPercentile for sample PGO.

Summary:
This performs better for sample PGO.
NFC as PGSOColdCodeOnlyForSamplePGO is still true.

Reviewers: davidxl

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75550