[ARM] Correct SP/PC handling in t2MOVr

PC isn't allowed in the source operand of t2MOVr, so change the register class
to one without PC. SP handling is slightly trickier and changes depending on if
we're in ARMv8, so do that in checkTargetMatchPredicate.

Differential Revision: https://reviews.llvm.org/D30199

llvm-svn: 295732