[llvm] [NFCI][MC][DecoderEmitter] Fix BitWidth for fixed length inst encodings (PR #154934)

[Rahul Joshi via llvm-commits]

jurahul wrote:

> > It may be something easy to fix upstream, but I'd rather make this a warning and give time to folks (upstream and downstream) to fix and then make it an error (if Size * 8 != Inst field size). WDYT?
> 
> Sounds reasonable

Thanks, I am thinking of keeping this as is, and then adding the warning as a separate PR.

https://github.com/llvm/llvm-project/pull/154934