[ARM][LowOverheadLoops] Fix generated code for "revert".
- t2CMPri shouldn't use CPSR if it isn't predicated. This doesn't
really have any visible effect at the moment, but it might matter in the
- The t2CMPri generated for t2WhileLoopStart might need to use a
register that isn't LR.
My team found this because we have a patch to track register liveness
late in the pass pipeline. I'll look into upstreaming it to help catch
issues like this earlier.
Differential Revision: https://reviews.llvm.org/D66243