[PGO][PGSO] SizeOpts changes.

Summary:
(Split of off D67120)

SizeOpts/MachineSizeOpts changes for profile guided size optimization.

(A second try after previously committed as r375254 and reverted as r375375.)

Subscribers: mgorny, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69409