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

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

It feels like ARM is easy to fix. Just don't underive Encoding16 from Encoding and provide its own Inst/SoftFail:
```
class Encoding16 {
  field bits<16> Inst;
  field bits<16> Unpredictable = 0;
  field bits<16> SoftFail = Unpredictable;
}
```
Assuming that the downstream targets are just as easy to fix, can we require Inst and Size to always match?


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