[NFC] Make AMDGPUCombinerHelper methods const

This is a follow-up to a previous commit (ee7ca0d) which eliminated
several "TODO: make CombinerHelper methods const" remarks. As promised
in that ealier commit, this change completes the set by also making
the methods of AMDGPUCombinerHelper const so that the Helper member of
AMDGPUPreLegalizerCombinerImpl can be const rather than explicitly
mutable.

Author: paulhuggett

llvm/lib/Target/AMDGPU/AMDGPUCombinerHelper.cpp

@@ -190,7 +190,7 @@ static unsigned inverseMinMax(unsigned Opc) {
 }
 
 bool AMDGPUCombinerHelper::matchFoldableFneg(MachineInstr &MI,
-                                             MachineInstr *&MatchInfo) {
+                                             MachineInstr *&MatchInfo) const {
   Register Src = MI.getOperand(1).getReg();
   MatchInfo = MRI.getVRegDef(Src);
 
@@ -259,7 +259,7 @@ bool AMDGPUCombinerHelper::matchFoldableFneg(MachineInstr &MI,
 }
 
 void AMDGPUCombinerHelper::applyFoldableFneg(MachineInstr &MI,
-                                             MachineInstr *&MatchInfo) {
+                                             MachineInstr *&MatchInfo) const {
   // Transform:
   // %A = inst %Op1, ...
   // %B = fneg %A
@@ -418,7 +418,7 @@ static bool isFPExtFromF16OrConst(const MachineRegisterInfo &MRI,
 bool AMDGPUCombinerHelper::matchExpandPromotedF16FMed3(MachineInstr &MI,
                                                        Register Src0,
                                                        Register Src1,
-                                                       Register Src2) {
+                                                       Register Src2) const {
   assert(MI.getOpcode() == TargetOpcode::G_FPTRUNC);
   Register SrcReg = MI.getOperand(1).getReg();
   if (!MRI.hasOneNonDBGUse(SrcReg) || MRI.getType(SrcReg) != LLT::scalar(32))
@@ -431,7 +431,7 @@ bool AMDGPUCombinerHelper::matchExpandPromotedF16FMed3(MachineInstr &MI,
 void AMDGPUCombinerHelper::applyExpandPromotedF16FMed3(MachineInstr &MI,
                                                        Register Src0,
                                                        Register Src1,
-                                                       Register Src2) {
+                                                       Register Src2) const {
   // We expect fptrunc (fpext x) to fold out, and to constant fold any constant
   // sources.
   Src0 = Builder.buildFPTrunc(LLT::scalar(16), Src0).getReg(0);

llvm/lib/Target/AMDGPU/AMDGPUCombinerHelper.h

@@ -23,13 +23,13 @@ class AMDGPUCombinerHelper : public CombinerHelper {
 public:
   using CombinerHelper::CombinerHelper;
 
-  bool matchFoldableFneg(MachineInstr &MI, MachineInstr *&MatchInfo);
-  void applyFoldableFneg(MachineInstr &MI, MachineInstr *&MatchInfo);
+  bool matchFoldableFneg(MachineInstr &MI, MachineInstr *&MatchInfo) const;
+  void applyFoldableFneg(MachineInstr &MI, MachineInstr *&MatchInfo) const;
 
   bool matchExpandPromotedF16FMed3(MachineInstr &MI, Register Src0,
-                                   Register Src1, Register Src2);
+                                   Register Src1, Register Src2) const;
   void applyExpandPromotedF16FMed3(MachineInstr &MI, Register Src0,
-                                   Register Src1, Register Src2);
+                                   Register Src1, Register Src2) const;
 };
 
 } // namespace llvm

llvm/lib/Target/AMDGPU/AMDGPUPreLegalizerCombiner.cpp

@@ -45,8 +45,7 @@ class AMDGPUPreLegalizerCombinerImpl : public Combiner {
 protected:
   const AMDGPUPreLegalizerCombinerImplRuleConfig &RuleConfig;
   const GCNSubtarget &STI;
-  // TODO: Make CombinerHelper methods const.
-  mutable AMDGPUCombinerHelper Helper;
+  const AMDGPUCombinerHelper Helper;
 
 public:
   AMDGPUPreLegalizerCombinerImpl(