[AMDGPU] Factored out common part of GCNRPTracker::reset()

lib/Target/AMDGPU/GCNRegPressure.h

protected:
   const LiveIntervals &LIS;
   LiveRegSet LiveRegs;
   GCNRegPressure CurPressure, MaxPressure;
   const MachineInstr *LastTrackedMI = nullptr;
   mutable const MachineRegisterInfo *MRI = nullptr;
 
   GCNRPTracker(const LiveIntervals &LIS_) : LIS(LIS_) {}
 
+  void reset(const MachineInstr &MI, const LiveRegSet *LiveRegsCopy,
+             bool After);
+
 public:
   // live regs for the current state
   const decltype(LiveRegs) &getLiveRegs() const { return LiveRegs; }
   const MachineInstr *getLastTrackedMI() const { return LastTrackedMI; }
 
   void clearMaxPressure() { MaxPressure.clear(); }
 
   // returns MaxPressure, resetting it

lib/Target/AMDGPU/GCNRegPressure.cpp

if (!LIS.hasInterval(Reg))
       continue;
     auto LiveMask = getLiveLaneMask(Reg, SI, LIS, MRI);
     if (LiveMask.any())
       LiveRegs[Reg] = LiveMask;
   }
   return LiveRegs;
 }
 
-void GCNUpwardRPTracker::reset(const MachineInstr &MI,
-                               const LiveRegSet *LiveRegsCopy) {
-  MRI = &MI.getParent()->getParent()->getRegInfo();
+void GCNRPTracker::reset(const MachineInstr &MI,
+                         const LiveRegSet *LiveRegsCopy,
+                         bool After) {
+  const MachineFunction &MF = *MI.getMF();
+  MRI = &MF.getRegInfo();
   if (LiveRegsCopy) {
     if (&LiveRegs != LiveRegsCopy)
       LiveRegs = *LiveRegsCopy;
   } else {
-    LiveRegs = getLiveRegsAfter(MI, LIS);
+    LiveRegs = After ? getLiveRegsAfter(MI, LIS)
+                     : getLiveRegsBefore(MI, LIS);
   }
+
   MaxPressure = CurPressure = getRegPressure(*MRI, LiveRegs);
 }
 
+void GCNUpwardRPTracker::reset(const MachineInstr &MI,
+                               const LiveRegSet *LiveRegsCopy) {
+  GCNRPTracker::reset(MI, LiveRegsCopy, true);
+}
+
 void GCNUpwardRPTracker::recede(const MachineInstr &MI) {
   assert(MRI && "call reset first");
 
   LastTrackedMI = &MI;
 
   if (MI.isDebugInstr())
     return;
 
bool GCNDownwardRPTracker::reset(const MachineInstr &MI,
                                  const LiveRegSet *LiveRegsCopy) {
   MRI = &MI.getParent()->getParent()->getRegInfo();
   LastTrackedMI = nullptr;
   MBBEnd = MI.getParent()->end();
   NextMI = &MI;
   NextMI = skipDebugInstructionsForward(NextMI, MBBEnd);
   if (NextMI == MBBEnd)
     return false;
-  if (LiveRegsCopy) {
-    if (&LiveRegs != LiveRegsCopy)
-      LiveRegs = *LiveRegsCopy;
-  } else {
-    LiveRegs = getLiveRegsBefore(*NextMI, LIS);
-  }
-  MaxPressure = CurPressure = getRegPressure(*MRI, LiveRegs);
+  GCNRPTracker::reset(*NextMI, LiveRegsCopy, false);
   return true;
 }
 
 bool GCNDownwardRPTracker::advanceBeforeNext() {
   assert(MRI && "call reset first");
 
   NextMI = skipDebugInstructionsForward(NextMI, MBBEnd);
   if (NextMI == MBBEnd)