[PGO][PGSO] Tune flags for profile guided size optimization.

Summary:
Tune the profile threshold flag value for instrumentation PGO based on internal
benchmarks.

Also, add flags to allow profile guided size optimizations for non-cold code
to be enabled separately for instrumentation and sample PGSO.

Neither changes the default behavior (yet) as it's disabled for non-cold code.

Reviewers: davidxl

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72937