Further tighten AArch32 push/pop selection.

Notably:

- Check IT block status for pops that branch (by loading to pc).
- Enforce register selection rules regarding sp, lr and pc. These differ
  between T32 and A32, and sometimes between encodings.
- Relax restrictions for A1 pop (multiple); a single register is
  permitted here.

Change-Id: Ic5fe640c957652dfe2deb9d244de07a9460f1c1c