swift234

[loop-arc] Refactor the dataflow rc state visitors so that they can be used with ARCRegionStates and ARCBBStates.

I am going to be replacing ARCBBState with a new class called ARCRegionState
that is essentially the same except it uses Regions instead of blocks. It
additionally has some refactorings that seem obvious to me now that I am looking
at the code.

rdar://22238658

Swift SVN r32722