[ARM] Recognize t2LoopEnd for software pipelining

- Add t2LoopEnd to TargetInstrInfo::analyzeBranch and
  related functions.  As there are many side effects of
  analyzing a branch, only do so if software pipelining
  is enabled to maintain previous behavior when pipelining
  is not desired.
- Make sure that t2LoopEndDec is immediately followed by
  a t2B when it is synthesized from a t2LoopEnd. This is
  done because the t2LoopEnd might have acquired a
  fall-through path, but IfConversion assumes that
  fall-through are only possible on analyzable branches.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D126322