[MachineCombiner] Add `const` to `shouldReduceRegisterPressure` arguments. NFC

[shchenz]

LGTM

llvm/include/llvm/CodeGen/TargetInstrInfo.h

public:
   virtual bool
   getMachineCombinerPatterns(MachineInstr &Root,
                              SmallVectorImpl<MachineCombinerPattern> &Patterns,
                              bool DoRegPressureReduce) const;
 
   /// Return true if target supports reassociation of instructions in machine
   /// combiner pass to reduce register pressure for a given BB.
   virtual bool
-  shouldReduceRegisterPressure(MachineBasicBlock *MBB,
-                               RegisterClassInfo *RegClassInfo) const {
+  shouldReduceRegisterPressure(const MachineBasicBlock *MBB,
+                               const RegisterClassInfo *RegClassInfo) const {
     return false;
   }
 
   /// Fix up the placeholder we may add in genAlternativeCodeSequence().
   virtual void
   finalizeInsInstrs(MachineInstr &Root, MachineCombinerPattern &P,
                     SmallVectorImpl<MachineInstr *> &InsInstrs) const {}
 

llvm/lib/Target/PowerPC/PPCInstrInfo.h

#endif
   bool getMachineCombinerPatterns(MachineInstr &Root,
                                   SmallVectorImpl<MachineCombinerPattern> &P,
                                   bool DoRegPressureReduce) const override;
 
   /// On PowerPC, we leverage machine combiner pass to reduce register pressure
   /// when the register pressure is high for one BB.
   /// Return true if register pressure for \p MBB is high and ABI is supported
   /// to reduce register pressure. Otherwise return false.
-  bool
-  shouldReduceRegisterPressure(MachineBasicBlock *MBB,
-                               RegisterClassInfo *RegClassInfo) const override;
+  bool shouldReduceRegisterPressure(
+      const MachineBasicBlock *MBB,
+      const RegisterClassInfo *RegClassInfo) const override;
 
   /// Fixup the placeholders we put in genAlternativeCodeSequence() for
   /// MachineCombiner.
   void
   finalizeInsInstrs(MachineInstr &Root, MachineCombinerPattern &P,
                     SmallVectorImpl<MachineInstr *> &InsInstrs) const override;
 
   bool isAssociativeAndCommutative(const MachineInstr &Inst) const override;

llvm/lib/Target/PowerPC/PPCInstrInfo.cpp

void PPCInstrInfo::finalizeInsInstrs(
   Register LoadNewConst =
       generateLoadForNewConst(ConstPoolIdx, &Root, C->getType(), InsInstrs);
 
   // Fill the placeholder with the new load from constant pool.
   Placeholder->setReg(LoadNewConst);
 }
 
 bool PPCInstrInfo::shouldReduceRegisterPressure(
-    MachineBasicBlock *MBB, RegisterClassInfo *RegClassInfo) const {
+    const MachineBasicBlock *MBB, const RegisterClassInfo *RegClassInfo) const {
 
   if (!EnableFMARegPressureReduction)
     return false;
 
   // Currently, we only enable register pressure reducing in machine combiner
   // for: 1: PPC64; 2: Code Model is Medium; 3: Power9 which also has vector
   // support.
   //
   // So we need following instructions to access a TOC entry:
   //
   // %6:g8rc_and_g8rc_nox0 = ADDIStocHA8 $x2, %const.0
   // %7:vssrc = DFLOADf32 target-flags(ppc-toc-lo) %const.0,
   //   killed %6:g8rc_and_g8rc_nox0, implicit $x2 :: (load 4 from constant-pool)
   //
   // FIXME: add more supported targets, like Small and Large code model, PPC32,
   // AIX.
   if (!(Subtarget.isPPC64() && Subtarget.hasP9Vector() &&
         Subtarget.getTargetMachine().getCodeModel() == CodeModel::Medium))
     return false;
 
   const TargetRegisterInfo *TRI = &getRegisterInfo();
-  MachineFunction *MF = MBB->getParent();
-  MachineRegisterInfo *MRI = &MF->getRegInfo();
+  const MachineFunction *MF = MBB->getParent();
+  const MachineRegisterInfo *MRI = &MF->getRegInfo();
 
-  auto GetMBBPressure = [&](MachineBasicBlock *MBB) -> std::vector<unsigned> {
+  auto GetMBBPressure =
+      [&](const MachineBasicBlock *MBB) -> std::vector<unsigned> {
     RegionPressure Pressure;
     RegPressureTracker RPTracker(Pressure);
 
     // Initialize the register pressure tracker.
     RPTracker.init(MBB->getParent(), RegClassInfo, nullptr, MBB, MBB->end(),
                    /*TrackLaneMasks*/ false, /*TrackUntiedDefs=*/true);
 
-    for (MachineBasicBlock::iterator MII = MBB->instr_end(),
-                                     MIE = MBB->instr_begin();
-         MII != MIE; --MII) {
-      MachineInstr &MI = *std::prev(MII);
+    for (const auto &MI : reverse(*MBB)) {
       if (MI.isDebugValue() || MI.isDebugLabel())
         continue;
       RegisterOperands RegOpers;
       RegOpers.collect(MI, *TRI, *MRI, false, false);
       RPTracker.recedeSkipDebugValues();
       assert(&*RPTracker.getPos() == &MI && "RPTracker sync error!");
       RPTracker.recede(RegOpers);
     }