[ARM] Expand predecessor search to multiple blocks when reverting WhileLoopStarts

We were previously only searching a single preheader for call
instructions when reverting WhileLoopStarts to DoLoopStarts. This
extends that to multiple blocks that can come up when, for example a
loop is expanded from a memcpy. It also expends the instructions from
just Call's to also include other LoopStarts, to catch other low
overhead loops in the preheader.

Differential Revision: https://reviews.llvm.org/D102269