RegisterPressure: A utility for computing register pressure within a

MachineInstr sequence.

This uses the new target interface for tracking register pressure
using pressure sets to model overlapping register classes and
subregisters.

RegisterPressure results can be tracked incrementally or stored at
region boundaries. Global register pressure can be deduced from local
RegisterPressure results if desired.

This is an early, somewhat untested implementation. I'm working on
testing it within the context of a register pressure reducing
MachineScheduler.

llvm-svn: 155454