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

[Craig Topper via llvm-commits]

================
@@ -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) {
----------------
topperc 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;
```

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