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

[Pengcheng Wang via llvm-commits]

================
@@ -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
----------------
wangpc-pp wrote:

This can be done automatically:
```
class GPRRegisterClassWithDiagnostic<dag regList>: GPRRegisterClass<regList> {
  let DiagnosticType= "InvalidRegClass" # NAME;
}
def GPRX1 : GPRRegisterClassWithDiagnostic<(add X1)>;
```
And finally we can remove `GPRRegisterClassWithDiagnostic` and move override of `DiagnosticType` to a base class.

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