Added -misched-regpressure option.

Register pressure tracking is half the complexity of the
scheduler. It's useful to be able to turn it off for compile time and
performance comparisons.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189987 91177308-0d34-0410-b5e6-96231b3b80d8
mi-sched: bypass heuristic checks when regpressure tracking is disabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189988 91177308-0d34-0410-b5e6-96231b3b80d8
mi-sched: Reuse an invalid HazardRecognizer to save compile time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189989 91177308-0d34-0410-b5e6-96231b3b80d8
mi-sched: Load clustering is a bit to expensive to enable unconditionally.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189990 91177308-0d34-0410-b5e6-96231b3b80d8
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
80 columns

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189992 91177308-0d34-0410-b5e6-96231b3b80d8
mi-sched: DEBUG cleanup, call tracePick for unidirectional scheduling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189993 91177308-0d34-0410-b5e6-96231b3b80d8
-view-misched-dags, better pruning.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189994 91177308-0d34-0410-b5e6-96231b3b80d8
Remove dead subtree limit code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189995 91177308-0d34-0410-b5e6-96231b3b80d8
comment typo

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Expand and rewrite comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189998 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r189902 as the workaround shouldn't be necessary anymore.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189999 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "Revert r189902 as the workaround shouldn't be necessary anymore."

Needs testcase updates.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190000 91177308-0d34-0410-b5e6-96231b3b80d8