[ARM][LowOverheadLoops] Change predicate inspection

Use the already provided helper function to get the operand type so
that we can detect whether the vpr is being used as a predicate or
not. Also use existing helpers to get the predicate indices when we
converting the vpt blocks. This enables us to support both types of
vpr predicate operand.

Differential Revision: https://reviews.llvm.org/D72504