[PGO][PGSO] Prep for enabling non-cold code size opts under non-partial-profile sample PGO.

Summary:
- Distinguish between partial-profile and non-partial-profile sample PGO.
- Add a flag for partial-profile sample PGO.
- Tune the sample PGO cutoff.
- No default behavior change (yet).

Reviewers: davidxl

Subscribers: eraman, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78949