[ARM] Fix Thumb2 compare being emitted ExpandCMP_SWAP

Given a patch like D129506, using instructions not valid for the current
target feature set becomes an error. This fixes an issue in
ARMExpandPseudo::ExpandCMP_SWAP where Thumb2 compares were used in
Thumb1Only code, such as thumbv8m.baseline targets.

Differential Revision: https://reviews.llvm.org/D129695