[ARM][LowOverheadLoops] Merge a VCMP and the new VPST into a VPT

There were cases where a VCMP and a VPST were merged even if the VCMP
didn't have the same defs of its operands as the VPST. This is fixed by
adding RDA checks for the defs. This however gave rise to cases where
the new VPST created would precede the un-merged VCMP and so would fail
a predicate mask assertion since the VCMP wasn't predicated. This was
solved by converting the VCMP to a VPT instead of inserting the new
VPST.

Differential Revision: https://reviews.llvm.org/D90461