MI-Sched: Track multiple candidates with the same priority level.

This eliminates the MultiPressure scheduling "reason". It was
sensitive to queue order. We don't like being sensitive to queue
order.
MI-Sched: Remove another heuristic that is sensitive to queue order.
MI Sched: fix a typo in RegPressure heuristics.
Remove compareRPDelta.

A complex, expensive heuristic with little value in the current design.
Give RegMax higher priority.
Reenable, improve, and add MI-Sched unit tests.
MI-Sched: handle ReadAdvance latencies as used by Swift.