mi-sched: improve the generic register pressure comparison.

Only compare pressure within the same set. When multiple sets are
affected, we prioritize the most constrained set.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189641 91177308-0d34-0410-b5e6-96231b3b80d8
Replace LiveInterval::killedAt with isKilledAtInstr.

Return true for LRGs that end at EarlyClobber or Register slots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189642 91177308-0d34-0410-b5e6-96231b3b80d8
mi-sched: update PressureDiffs on-the-fly for liveness.

This removes all expensive pressure tracking logic from the scheduling
critical path of node comparison.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189643 91177308-0d34-0410-b5e6-96231b3b80d8