[llvm] [RISCV][NFC] Combine redundant 'if' statements (PR #70423)

[via llvm-commits]

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@@ -257,15 +248,12 @@ static DecodeStatus decodeVMaskReg(MCInst &Inst, uint64_t RegNo,
                                    uint64_t Address,
                                    const MCDisassembler *Decoder) {
   MCRegister Reg = RISCV::NoRegister;
-  switch (RegNo) {
-  default:
+  if (RegNo > 2) {
     return MCDisassembler::Fail;
-  case 0:
+  } else if (RegNo == 0) {
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knightXun wrote:

> I meant.
> 
> ```
> if (RegNo > 2)
>   return MCDisassembler::Fail;
> if (RegNo == 0) <- no need for else before this.
>   Reg = RISCV::V0
> ```
> 
> Though reading the code again. I think this is simpler
> 
> ```
> if (RegNo > 2)
>   return MCDisassembler::Fail;
> MCRegister Reg = (RegNo == 0) ? RISCV::V0 : RISCV::NoRegister;
> ```

that's great!

https://github.com/llvm/llvm-project/pull/70423