mi-sched: Suppress register pressure tracking when the scheduling window is too small.

mi-sched: Suppress register pressure tracking when the scheduling window is too small.

If the instruction window is < NumRegs/2, pressure tracking is not
likely to be effective. The scheduler has to process a very large
number of tiny blocks. We want this to be fast.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189991 91177308-0d34-0410-b5e6-96231b3b80d8