[ARM] Accept a subset of Thumb GPR register class when emitting an SP-relative

load instruction

The function `Thumb1InstrInfo::loadRegFromStackSlot` accepts only the `tGPR`
register class. The function serves to emit a `tLDRspi` instruction and
certainly any subset of the `tGPR` register class is a valid destination of the
load.

Differential revision: https://reviews.llvm.org/D42535

llvm-svn: 323514