mi-sched: Don't call MBB.size() in initSUnits. The driver already has instr count.

This fixes a pathological compile time problem with very large blocks
and lots of scheduling boundaries.
Remove unused field.
MI Sched: record local vreg uses.

This will be used to compute the cyclic critical path and to
update precomputed per-node pressure differences.
In the longer term, it could also be used to speed up LiveInterval
update by avoiding visiting all global vreg users.
Fix indentation
Adds cyclic critical path computation and heuristics, temporarily disabled.

Estimate the cyclic critical path within a single block loop. If the
acyclic critical path is longer, then the loop will exhaust OOO
resources after some number of iterations. If lag between the acyclic
critical path and cyclic critical path is longer the the time it takes
to issue those loop iterations, then aggressively schedule for
latency.
Add a convenient PSetIterator for visiting pressure sets affected by a register.
Simplify RegPressure helpers.
Rename to RegPressure API parameters RegUnits.
PrintVRegOrUnit