[AMDGPU][Scheduler] Scoring system for rematerializations (#175050)

This is a significant refactoring of the scheduler's rematerialization
stage meant to improve rematerialization capabilities and lay strong
foundations for future improvements.

As before, the stage identifies scheduling regions in which RP must be
reduced (so-called "target regions"), then rematerializes registers to
try and achieve the desired reduction. All regions affected by
rematerializations are re-scheduled, and, if the MIR is deemed worse
than before, rematerializations are rolled back to leave the MIR in its
pre-stage state.

The core contribution is a scoring system to estimate the benefit of
each rematerialization candidate. This score favors rematerializing
candidates which, in order, would

1. (if the function is spilling) reduce RP in highest-frequency target
regions,
2. be rematerialized to lowest-frequency target regions, and
3. reduce RP in the highest number of target regions.

All rematerialization opportunities are initially scored and
rematerialized in decreasing score order until RP objectives are met or
pre-computed scores diverge from reality; in the latter case remaining
candidates are re-scored and the process repeats. New tests in
`machine-scheduler-rematerialization-scoring.mir` showcase how the
scoring system dictates which rematerialization are the most beneficial
and therefore performed first

A minor contribution included in this PR following previous feedback is
that rollback now happens in-place i.e., without having to re-create the
rematerialized MI. This leaves original slot indices and registers
untouched. We achieve this by temporarily switching the opcode of
rollback-able instructions to a debug opcode during re-scheduling so
that they are ignored.