(temp): begin typdeffing Register
(temp): make RegisterMask a class
(temp): catch arm up
(temp): begin renaming lir:: types
(temp): finish renaming lir:: types (fixup arm)
(temp): rename lir::Register to lir::RegisterPair
(temp): make Register a class
(temp): finish making Register a class (fixup arm)
(temp): add back NoRegister and remove some implicit int->Register casts
(temp): transition x86 registers to Register instances
(temp): begin getting rid of implicit Register casts (x86)
(temp): further get rid of implicit Register casts (x86)