[ARM] Revert WhileLoopStartLR to DoLoopStart

If a WhileLoopStartLR is reverted due to calls in the preheader, we may
still be able to instead create a DoLoopStart, preserving the low
overhead loop. This adds code for that, only reverting the
WhileLoopStartR to a Br/Cmp, leaving the rest of the low overhead loop
in place.

Differential Revision: https://reviews.llvm.org/D98413