Clear RP trackers at beginning of analysis. (llvm#777)

Author: jakedaly-amd

llvm/lib/Target/AMDGPU/AMDGPURegPressAnalysis.cpp

@@ -37,6 +37,11 @@ AMDGPURegPressAnalysis::getPressure(const MachineBasicBlock *MBB) const {
 }
 
 bool AMDGPURegPressAnalysis::runOnMachineFunction(MachineFunction &MF) {
+
+  // Reset trackers, or else they will carry across functions in a module.
+  BlockPressure.clear();
+  MaxPressure.clear();
+
   if (skipFunction(MF.getFunction()))
     return false;