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

[Sergei Barannikov via llvm-commits]

s-barannikov 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

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