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

[Wang Pengcheng via llvm-commits]

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@@ -278,21 +278,7 @@ class RVVRegisterRegAlloc : public RegisterRegAllocBase<RVVRegisterRegAlloc> {
 
 static bool onlyAllocateRVVReg(const TargetRegisterInfo &TRI,
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wangpc-pp wrote:

Yes, it has prototype `llvm/include/llvm/CodeGen/RegAllocCommon.h::RegClassFilterFunc`.

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