Add a subtarget hook: enablePostMachineScheduler.

As requested by AArch64 subtargets.

Note that this will have no effect until the
AArch64 target actually enables the pass like this:
substitutePass(&PostRASchedulerID, &PostMachineSchedulerID);

As soon as armv7 switches over, PostMachineScheduler will become the
default postRA scheduler, so this won't be necessary any more.
Targets using the old postRA schedule would then do:
substitutePass(&PostMachineSchedulerID, &PostRASchedulerID);

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210167 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineScheduler.cpp

@@ -333,6 +333,12 @@ bool PostMachineScheduler::runOnMachineFunction(MachineFunction &mf) {
   if (skipOptnoneFunction(*mf.getFunction()))
     return false;
 
+  const TargetSubtargetInfo &ST =
+    mf.getTarget().getSubtarget<TargetSubtargetInfo>();
+  if (!ST.enablePostMachineScheduler()) {
+    DEBUG(dbgs() << "Subtarget disables post-MI-sched.\n");
+    return false;
+  }
   DEBUG(dbgs() << "Before post-MI-sched:\n"; mf.print(dbgs()));
 
   // Initialize the context of the pass.