[GISel][CSE][NFC]: Handle mutual recursion when inserting node

[aditya_nandakumar]

Summary

GISel's CSE mechanism lazily inserts instructions into the CSE List to improve on efficiency as well as efficacy of CSE (for allowing partially built instructions to be fully built). There's unfortunately a mutual recursion via
handleRecordedInsts -> handleRecordedInst -> insertNode-> handleRecordedInsts.
So this change simply records that we're already draining this list so we can just bail out on the recursion.
No changes to codegen are expected as we're still draining/handling the temporary list via pop_back and we should get the same sequence of instructions whether we call pop_back in a loop at the top level or recursive.

[dsanders]

LGTM

llvm/include/llvm/CodeGen/GlobalISel/CSEInfo.h

class GISelCSEInfo : public GISelChangeObserver {
   MachineInstr *getMachineInstrIfExists(FoldingSetNodeID &ID,
                                         MachineBasicBlock *MBB,
                                         void *&InsertPos);
 
   /// Use this method to allocate a new UniqueMachineInstr for MI and insert it
   /// into the CSEMap. MI should return true for shouldCSE(MI->getOpcode())
   void insertInstr(MachineInstr *MI, void *InsertPos = nullptr);
 
+  bool HandlingRecordedInstrs = false;
+
 public:
   GISelCSEInfo() = default;
 
   virtual ~GISelCSEInfo();
 
   void setMF(MachineFunction &MF);
 
   Error verify();

llvm/lib/CodeGen/GlobalISel/CSEInfo.cpp

void GISelCSEInfo::handleRemoveInst(MachineInstr *MI) {
   if (auto *UMI = InstrMapping.lookup(MI)) {
     invalidateUniqueMachineInstr(UMI);
     InstrMapping.erase(MI);
   }
   TemporaryInsts.remove(MI);
 }
 
 void GISelCSEInfo::handleRecordedInsts() {
+  if (HandlingRecordedInstrs)
+    return;
+  HandlingRecordedInstrs = true;
   while (!TemporaryInsts.empty()) {
     auto *MI = TemporaryInsts.pop_back_val();
     handleRecordedInst(MI);
   }
+  HandlingRecordedInstrs = false;
 }
 
 bool GISelCSEInfo::shouldCSE(unsigned Opc) const {
   assert(CSEOpt.get() && "CSEConfig not set");
   return CSEOpt->shouldCSEOpc(Opc);
 }
 
 void GISelCSEInfo::erasingInstr(MachineInstr &MI) { handleRemoveInst(&MI); }