[ARM] Enable spilling of the hGPR register class in Thumb2

Generalize code in Thumb2InstrInfo::storeRegToStackSlot() and
loadRegToStackSlot() to allow the GPR class or any of its sub-classes
(including hGPR) to be stored/loaded by ARM::t2STRi12/ARM::t2LDRi12.

Differential Revision: https://reviews.llvm.org/D51927

llvm-svn: 346401