InstrProf: Stop using RegionCounter outside of CodeGenPGO (NFC)

InstrProf: Stop using RegionCounter outside of CodeGenPGO (NFC)

The RegionCounter type does a lot of legwork, but most of it is only
meaningful within the implementation of CodeGenPGO. The uses elsewhere
in CodeGen generally just want to increment or read counters, so do
that directly.

llvm-svn: 235664