[RegAllocGreedy] New hook regClassPriorityTrumpsGlobalness

Add a new TargetRegisterInfo hook to allow targets to tweak the
priority of live ranges, so that AllocationPriority of the register
class will be treated as more important than whether the range is local
to a basic block or global. This is determined per-MachineFunction.

Differential Revision: https://reviews.llvm.org/D125102