[ARM] Make narrow push/pop in SEH prologues/epilogues where applicable

We intentionally disable Thumb2SizeReduction for SEH
prologues/epilogues, to avoid needing to guess what will happen with
the instructions in a potential future pass in frame lowering.

But for this specific case, where we know we can express the
intent with a narrow instruction, change to that instruction form
directly in frame lowering.

Differential Revision: https://reviews.llvm.org/D126948