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

[Rahul Joshi via llvm-commits]

jurahul wrote:

@topperc Please let me know if this new scheme looks ok. If yes, I'll migrate the rest of the targets (Right now I just changed AARCH64 and RISCV) to use this, and add some unit tests for a final review.

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