swift234

[loop-arc] Rename LoopRegionID => SubregionID. NFC.

In order to handle non-local loop region edges, I will need to introduce
a SuccessorID type. Having a type named LoopRegionID and another type
named SuccessorID is confusing, so it makes sense to give LoopRegionID a
name that matches its use case.

Swift SVN r32643