MachineScheduler: Add a target hook for deciding which RegPressure sets to

increase

Summary:
This patch adds a function called getRegPressureSetScore() to
TargetRegisterInfo.  The MachineScheduler uses this when comparing
instruction that increase the register pressure of different sets
to determine which set is safer to increase.

This hook is useful for GPU targets where the number of registers in the
class is not the best metric for determing which presser set is safer to
increase.

Future work may include adding more parameters to this function, like
for example, the current pressure level of the set or the amount that
the pressure will be increased/decreased.

Reviewers: qcolombet, escha, arsenm, atrick, MatzeB

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14806

llvm-svn: 255795