[llvm] [RISCV] Improve Errors for X1/X5/X1X5 Reg Classes (PR #126184)

[Sam Elliott via llvm-commits]

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@@ -247,7 +247,11 @@ def GPR : GPRRegisterClass<(add (sequence "X%u", 10, 17),
                                 (sequence "X%u", 0, 4))>;
 
 def GPRX0 : GPRRegisterClass<(add X0)>;
+
+let DiagnosticType = "InvalidRegClassGPRX1" in
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lenary wrote:

#126290 is a (NFC) PR to adopt DiagnosticString. I will rebase this change on that if/when it lands.

https://github.com/llvm/llvm-project/pull/126184