CodeGen: Remove some unnecessary braces
CodeGen: Remove a superfluous setCurrentRegionCount

This call looks like it was an artifact of an earlier change, and
doesn't actually make sense. We begin a new region immediately anyway,
so it was mostly harmless.
CodeGen: Introduce CodeGenPGO::setCurrentRegionUnreachable

There are a number of places where we do PGO.setCurrentRegionCount(0)
directly after an unconditional branch. Give this operation a name so
that it's clearer why we're doing this.
CodeGen: Rename adjustFallThroughCount -> adjustForControlFlow

adjustFallThroughCount isn't a good name, and the documentation was
even worse. This commit attempts to clarify what it's for and when to
use it.
CodeGen: Clarify a comment about PGO in case statement ranges