Added MachineSchedPolicy.

Allow subtargets to customize the generic scheduling strategy.
This is convenient for targets that don't need to add new heuristics
by specializing the strategy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190176 91177308-0d34-0410-b5e6-96231b3b80d8
mi-sched: Reorder Cyclicpath (latency) and CriticalMax (pressure) heuristics.

The latency based scheduling could induce spills in some cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190177 91177308-0d34-0410-b5e6-96231b3b80d8
mi-sched: register pressure update tracing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190178 91177308-0d34-0410-b5e6-96231b3b80d8
mi-sched: print tree size in -view-misched-dags

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190179 91177308-0d34-0410-b5e6-96231b3b80d8
mi-sched: improve regpressure tracing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190180 91177308-0d34-0410-b5e6-96231b3b80d8
mi-sched: cleanup register pressure update, remove a FIXME.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190181 91177308-0d34-0410-b5e6-96231b3b80d8