[llvm] [DecoderEmitter] Eliminate `DecodeComplete` parameter to `decodeToMCInst` (PR #159130)

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

In the end, I think there is no behavioral difference here.

```
Opc         Status  DecodeComplete  Result
Decode      Fail    true            return status
Decode      Success true            return status
TryDecode   Fail    false           leave scope
TryDecode   Success true            return status
```

We leave scope iff `TryDecode && DecodeComplete == false`, which is equivalent to `TryDecode && Status == Fail`.


https://github.com/llvm/llvm-project/pull/159130