MI Sched fix: assert "Disconnected LRG within the scheduling region."
MI Sched: Track live-thru registers.

When registers must be live throughout the scheduling region, increase
the limit for the register class. Once we exceed the original limit,
they will be spilled, and there's no point further reducing pressure.

This isn't a perfect heuristics but avoids a situation where the
scheduler could become trapped by trying to achieve the impossible.
whitespace
Down-scale slot index distance to save bits.
Remove dead code.