[ARM][LowOverheadLoops] Add LR def safety check

Converting the *LoopStart pseudo instructions into DLS/WLS results in
LR being defined. These instructions were inserted on the assumption
that LR would already contain the loop counter because a mov is
introduced during ISel as the the consumers in the loop can only use
LR. That assumption proved wrong!

So perform a safety check, finding an appropriate place to insert the
DLS/WLS instructions or revert if this isn't possible.

Differential Revision: https://reviews.llvm.org/D67539

llvm-svn: 372111