Tidy some register classes for ARM and Thumb

Tidy up three places where the register class for ARM and Thumb wasn't
restrictive enough:
 - No PC dest for reg-reg add/orr/sub.
 - No PC dest for shifts.
 - No PC or SP for Thumb2 reg-imm add.

I encountered this while combining FastISel with
-verify-machineinstrs. These instructions defined registers whose
classes weren't restrictive enough, and the uses failed
verification. They're also undefined in the ISA, or would produce code
that FastISel wouldn't want. This doesn't fix the register class
narrowing issue (where uses should restrict definitions), and isn't
thorough, but it's a small step in the right direction.