misched: Add register pressure backoff to ConvergingScheduler.

Prioritize the instruction that comes closest to keeping pressure
under the target's limit. Then prioritize instructions that avoid
increasing the max pressure in the scheduled region. The max pressure
heuristic is a tad aggressive. Later I'll fix it to consider the
unscheduled pressure as well.

WIP: This is mostly functional but untested and not likely to do much good yet.
llvm-svn: 156574