[PowerPC][NFC] Added `getDefMIPostRA` method so that other methods don't need to implement it repeatedly

[lkail]

Summary

In PostRA phase, we often have to find out the most recent definition of a register.
This patch adds getDefMIPostRA so that other methods can use it rather than implementing it repeatedly.

[shchenz]

LGTM.
Please fix the minor comments when commit this.

llvm/lib/Target/PowerPC/PPCInstrInfo.h

143

MI -> \p MI
Reg -> \p Reg

144

SeenIntermediate -> \p SeenIntermediateUse
is set -> is set to?
MI -> \p MI

[lkail]

FYI, I should have make getDefMIPostRA public. So I committed another NFC patch to make getDefMIPostRA public. https://reviews.llvm.org/rG985e52a4c1c6

llvm/lib/Target/PowerPC/PPCInstrInfo.h

class PPCInstrInfo : public PPCGenInstrInfo {
   // could be forwarded from some inst that feeds it. If yes, return the
   // Def of that operand. And OpNoForForwarding is the operand index in
   // the 'MI' for that 'Def'. If we see another use of this Def between
   // the Def and the MI, SeenIntermediateUse becomes 'true'.
   MachineInstr *getForwardingDefMI(MachineInstr &MI,
                                    unsigned &OpNoForForwarding,
                                    bool &SeenIntermediateUse) const;
 
+  // In PostRA phase, try to find instruction defines Reg before MI.
+  // SeenIntermediate is set true if uses between DefMI and MI exist.
+  MachineInstr *getDefMIPostRA(unsigned Reg, MachineInstr &MI,
+                               bool &SeenIntermediateUse) const;
+
   // Can the user MI have it's source at index \p OpNoForForwarding
   // forwarded from an add-immediate that feeds it?
   bool isUseMIElgibleForForwarding(MachineInstr &MI, const ImmInstrInfo &III,
                                    unsigned OpNoForForwarding) const;
   bool isDefMIElgibleForForwarding(MachineInstr &DefMI,
                                    const ImmInstrInfo &III,
                                    MachineOperand *&ImmMO,
                                    MachineOperand *&RegMO) const;

llvm/lib/Target/PowerPC/PPCInstrInfo.cpp

void PPCInstrInfo::replaceInstrWithLI(MachineInstr &MI,
   else
     MI.setDesc(get(LII.Is64Bit ? PPC::LI8 : PPC::LI));
 
   // Set the immediate.
   MachineInstrBuilder(*MI.getParent()->getParent(), MI)
       .addImm(LII.Imm);
 }
 
+MachineInstr *PPCInstrInfo::getDefMIPostRA(unsigned Reg, MachineInstr &MI,
+                                           bool &SeenIntermediateUse) const {
+  assert(!MI.getParent()->getParent()->getRegInfo().isSSA() &&
+         "Should be called after register allocation.");
+  const TargetRegisterInfo *TRI = &getRegisterInfo();
+  MachineBasicBlock::reverse_iterator E = MI.getParent()->rend(), It = MI;
+  It++;
+  SeenIntermediateUse = false;
+  for (; It != E; ++It) {
+    if (It->modifiesRegister(Reg, TRI))
+      return &*It;
+    if (It->readsRegister(Reg, TRI))
+      SeenIntermediateUse = true;
+  }
+  return nullptr;
+}
+
 MachineInstr *PPCInstrInfo::getForwardingDefMI(
   MachineInstr &MI,
   unsigned &OpNoForForwarding,
   bool &SeenIntermediateUse) const {
   OpNoForForwarding = ~0U;
   MachineInstr *DefMI = nullptr;
   MachineRegisterInfo *MRI = &MI.getParent()->getParent()->getRegInfo();
   const TargetRegisterInfo *TRI = &getRegisterInfo();
if (MRI->isSSA()) {
     // Don't convert or %X, %Y, %Y since that's just a register move.
     if ((Opc == PPC::OR || Opc == PPC::OR8) &&
         MI.getOperand(1).getReg() == MI.getOperand(2).getReg())
       return nullptr;
     for (int i = 1, e = MI.getNumOperands(); i < e; i++) {
       MachineOperand &MO = MI.getOperand(i);
       SeenIntermediateUse = false;
       if (MO.isReg() && MO.isUse() && !MO.isImplicit()) {
-        MachineBasicBlock::reverse_iterator E = MI.getParent()->rend(), It = MI;
-        It++;
         unsigned Reg = MI.getOperand(i).getReg();
-
+        // If we see another use of this reg between the def and the MI,
+        // we want to flat it so the def isn't deleted.
+        MachineInstr *DefMI = getDefMIPostRA(Reg, MI, SeenIntermediateUse);
+        if (DefMI) {
           // Is this register defined by some form of add-immediate (including
           // load-immediate) within this basic block?
-        for ( ; It != E; ++It) {
-          if (It->modifiesRegister(Reg, &getRegisterInfo())) {
-            switch (It->getOpcode()) {
-            default: break;
-            case PPC::LI:
-            case PPC::LI8:
-            case PPC::ADDItocL:
-            case PPC::ADDI:
-            case PPC::ADDI8:
-              OpNoForForwarding = i;
-              return &*It;
-            }
-            break;
-          } else if (It->readsRegister(Reg, &getRegisterInfo()))
-            // If we see another use of this reg between the def and the MI,
-            // we want to flat it so the def isn't deleted.
-            SeenIntermediateUse = true;
+          switch (DefMI->getOpcode()) {
+          default:
+            break;
+            return DefMI;
         }
       }
     }
   }
   return OpNoForForwarding == ~0U ? nullptr : DefMI;
 }
 
 const unsigned *PPCInstrInfo::getStoreOpcodesForSpillArray() const {