[llvm] [NFC][CodeGen] Add helper function to check SubReg validity (PR #181489)

[Matt Arsenault via llvm-commits]

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@@ -442,6 +442,14 @@ const TargetRegisterClass *TargetRegisterInfo::findCommonRegClass(
   return getCommonSubClass(DefRC, SrcRC);
 }
 
+bool TargetRegisterInfo::isSubRegValidForRegClass(const TargetRegisterClass *RC,
+                                                  unsigned Idx) const {
+  // `Idx` is valid if the largest subclass of `RC` that supports
+  // sub-register index `Idx` is same as `RC`. That is, every physical
+  // register in `RC` support sub-register index `Idx`.
+  return getSubClassWithSubReg(RC, Idx) == RC;
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arsenm wrote:

Can move this into the header 

https://github.com/llvm/llvm-project/pull/181489