[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

@@ -455,7 +455,7 @@ void AMDGPUCombinerHelper::applyExpandPromotedF16FMed3(MachineInstr &MI,
 
 bool AMDGPUCombinerHelper::matchCombineFmulWithSelectToFldexp(
     MachineInstr &MI, MachineInstr &Sel,
-    std::function<void(MachineIRBuilder &)> &MatchInfo) {
+    std::function<void(MachineIRBuilder &)> &MatchInfo) const {
   assert(MI.getOpcode() == TargetOpcode::G_FMUL);
   assert(Sel.getOpcode() == TargetOpcode::G_SELECT);
   assert(MI.getOperand(2).getReg() == Sel.getOperand(0).getReg());