[ARM] Use the correct opcodes for Thumb2 segmented stack frame lowering

The segmented stack lowering code appears to be using ARM opcodes under
Thumb2. The MRC opcode will be the same for Thumb and ARM, but t2LDR
seems wrong. Either way, using the correct thumb vs arm opcodes is more
correct.

Differential Revision: https://reviews.llvm.org/D72074