[llvm] [RegisterCoalescer]: Try inflated RC for coalescing reg->subreg (PR #134438)

[Matt Arsenault via llvm-commits]

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@@ -731,7 +731,14 @@ class MachineRegisterInfo {
 
   /// recomputeRegClass - Try to find a legal super-class of Reg's register
   /// class that still satisfies the constraints from the instructions using
-  /// Reg.  Returns true if Reg was upgraded.
+  /// \p Reg. \p return the super-class TargetRegisterClass if one was found,
+  /// otherwise \p return the original TargetRegisterClass.
+  const TargetRegisterClass *
+  getLargestConstrainedSuperClass(Register Reg) const;
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arsenm wrote:

Should remove the function name, this hasn't been necessary in doxygen for a long time.

Also should probably name this computeLargestConstrainedSuperClass? It's not simple get 

https://github.com/llvm/llvm-project/pull/134438