[llvm] Reland "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" (PR #168353)

[Benjamin Maxwell via llvm-commits]

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@@ -307,7 +307,12 @@ class RegisterCoalescer : private LiveRangeEdit::Delegate {
   /// number if it is not zero. If DstReg is a physical register and the
   /// existing subregister number of the def / use being updated is not zero,
   /// make sure to set it to the correct physical subregister.
-  void updateRegDefsUses(Register SrcReg, Register DstReg, unsigned SubIdx);
+  ///
+  /// If \p SubregToRegSrcInst is not empty, we are coalescing a
+  /// `DstReg = SUBREG_TO_REG SrcReg`, which should introduce an
+  /// implicit-def of DstReg on instructions that define SrcReg.
+  void updateRegDefsUses(Register SrcReg, Register DstReg, unsigned SubIdx,
+                         ArrayRef<MachineInstr *> SubregToRegSrcInst = {});
----------------
MacDue wrote:

```suggestion
                         ArrayRef<MachineInstr *> SubregToRegSrcInsts = {});
```

https://github.com/llvm/llvm-project/pull/168353