[llvm] [NFC][CodeGen] Rewrite comment for `getSubRegisterClass` (PR #182339)

[Rahul Joshi via llvm-commits]

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@@ -727,8 +727,13 @@ class LLVM_ABI TargetRegisterInfo : public MCRegisterInfo {
     return RC;
   }
 
-  /// Return a register class that can be used for a subregister copy from/into
-  /// \p SuperRC at \p SubRegIdx.
+  /// Returns the register class of all sub-registers of \p SuperRC obtained by
+  /// applying the sub-register index \p SubRegIdx.
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jurahul wrote:

I have not ignored it this, just haven't gotten around to find the answer.

https://github.com/llvm/llvm-project/pull/182339