[llvm] InstructionSelect: Use GISelChangeObserver instead of MachineFunction::Delegate (PR #105725)

[Tobias Stadler via llvm-commits]

================
@@ -75,19 +75,21 @@ InstructionSelect::InstructionSelect(CodeGenOptLevel OL, char &PassID)
 /// a non-obvious limitation for selector implementers. Therefore, to allow
 /// deletion of arbitrary instructions, we detect this case and continue
 /// selection with the predecessor of the deleted instruction.
-class InstructionSelect::MIIteratorMaintainer
-    : public MachineFunction::Delegate {
+class InstructionSelect::MIIteratorMaintainer : public GISelChangeObserver {
 #ifndef NDEBUG
   SmallSetVector<const MachineInstr *, 32> CreatedInstrs;
 #endif
 public:
   MachineBasicBlock::reverse_iterator MII;
 
-  void MF_HandleInsertion(MachineInstr &MI) override {
+  void changingInstr(MachineInstr &MI) override {}
+  void changedInstr(MachineInstr &MI) override {}
----------------
tobias-stadler wrote:

Please add `llvm_unreachable("InstructionSelect does not track changed instructions!");` or similar to changingInstr and changedInstr. We only install a Delegate instead of a full observer on purpose. We don't want this to be changed by accident and we don't want InstructionSelector implementations to start calling this thinking it does the right thing.

https://github.com/llvm/llvm-project/pull/105725