[AMDGPU] Remember to clear a DenseMap between runs of SIInsertWaitcnts

This caused nondeterministic codegen in some cases.

Author: jayfoad

llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp

@@ -2598,6 +2598,7 @@ bool SIInsertWaitcnts::runOnMachineFunction(MachineFunction &MF) {
     Modified = true;
   }
   ReleaseVGPRInsts.clear();
+  PreheadersToFlush.clear();
   SLoadAddresses.clear();
 
   return Modified;