[llvm] [CodeGen] Fix incorrect rematerialization rollback order (PR #197576)

[Matt Arsenault via llvm-commits]

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@@ -514,6 +511,12 @@ class Rematerializer {
 /// rematerializations as soon as it is attached to the rematerializer.
 class LLVM_ABI Rollbacker : public Rematerializer::Listener {
 public:
+  /// An insertion position pointer in the MIR.
+  union InsertPosPtr {
+    MachineInstr *MI;
+    MachineBasicBlock *MBB;
+  };
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arsenm wrote:

Why not MachineBasicBlock + MachineBasicBlock::iterator? There's probably already a utility for that 

https://github.com/llvm/llvm-project/pull/197576