MISched: Fix -misched-regpressure=0 if subreg liveness enabled

[arsenm]

Summary

Test is waiting on fixing several more crashes in the AMDGPU scheduler implementation with this

[arsenm]

ping

[qcolombet]

Looks reasonable!

[arsenm]

r362174

lib/CodeGen/MachineScheduler.cpp

void GenericScheduler::initPolicy(MachineBasicBlock::iterator Begin,
   // For generic targets, we default to bottom-up, because it's simpler and more
   // compile-time optimizations have been implemented in that direction.
   RegionPolicy.OnlyBottomUp = true;
 
   // Allow the subtarget to override default policy.
   MF.getSubtarget().overrideSchedPolicy(RegionPolicy, NumRegionInstrs);
 
   // After subtarget overrides, apply command line options.
-  if (!EnableRegPressure)
+  if (!EnableRegPressure) {
     RegionPolicy.ShouldTrackPressure = false;
+    RegionPolicy.ShouldTrackLaneMasks = false;
+  }
 
   // Check -misched-topdown/bottomup can force or unforce scheduling direction.
   // e.g. -misched-bottomup=false allows scheduling in both directions.
   assert((!ForceTopDown || !ForceBottomUp) &&
          "-misched-topdown incompatible with -misched-bottomup");
   if (ForceBottomUp.getNumOccurrences() > 0) {
     RegionPolicy.OnlyBottomUp = ForceBottomUp;
     if (RegionPolicy.OnlyBottomUp)