[llvm] [RISCV][NFC] Add helpers for RVV register classes (PR #84144)

[Luke Lau via llvm-commits]

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@@ -110,13 +110,31 @@ struct RISCVRegisterInfo : public RISCVGenRegisterInfo {
     return RC;
   }
 
-  bool doesRegClassHavePseudoInitUndef(
-      const TargetRegisterClass *RC) const override {
+  bool isRVVRegClass(const TargetRegisterClass *RC) const {
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lukel97 wrote:

Nit, `isRVVRegClass` sounds like it should include the segmented reg classes since they're a part of RVV too.  Maybe name it something like `isVRRegClass` and `isVRNRegClass`? 

https://github.com/llvm/llvm-project/pull/84144