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

[Rahul Joshi via llvm-commits]

jurahul wrote:

> > Currently, the code generated by decoder emitter always set DeocdeComplete to false when returning Fail from decodeToMCInst
> 
> But not when OpInfo.HasCompleteDecoder is false, right? So it seems like this change ought to break some tests...I'm surprised it doesn't.

Right, I see that either the whole instruction decoder may be incomplete, or only certain operands may be incomplete, in which case other (non-incomplete) operands may fail to decode terminally vs the incomplete operands may bail to decode with an incomplete signal. Yeah, none of the tests seems to break through. Maybe AArch64 does not exercise this case? 

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