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

[Rahul Joshi via llvm-commits]

jurahul wrote:

Looks like we started to use a new clang-format version upstream, so my local clang-format now not doing all the things expected.

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