[ARM][LowOverheadLoops] Convert intermediate vpr use assertion to condition

This converts the intermediate VPR use assertion to a condition in the if-statement to protect against assertion failures in case behaviuour is changed.

This is a follow-up to https://reviews.llvm.org/D90935 and implements the post-approval comments.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D91790