[llvm] [TableGen][DecoderEmitter] Add option to emit type-specialized `decodeToMCInst` (PR #146593)

[Rahul Joshi via llvm-commits]

jurahul wrote:

> > Many backends seems to choose the same type (for example uint64_t) for all their 16/32/48/64 bit insts.
> 
> I'm probably going to change to uint64_t for RISC-V. The 48-bit instructions are only used by one vendor and are relatively recent additions. I think the duplication cost just wasn't considered when they were added.
> 
> I agree adding to the Inst class might be too invasive. I still think it should be in .td files somehow. Needing to change a CMake file and replicating to GN and the other build systems when a new instruction width is added seems bad.

Right, is the option #3 above palatable? We essentially encode it as a standalone record that the DecoderEmitter will look for. 

https://github.com/llvm/llvm-project/pull/146593