[ARM] Add a RegAllocHint for hinting t2DoLoopStart towards LR

This hints the operand of a t2DoLoopStart towards using LR, which can
help make it more likely to become t2DLS lr, lr. This makes it easier to
move if needed (as the input is the same as the output), or potentially
remove entirely.

The hint is added after others (from COPY's etc) which still take
precedence. It needed to find a place to add the hint, which currently
uses the post isel custom inserter.

Differential Revision: https://reviews.llvm.org/D89883