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

[Rahul Joshi via llvm-commits]

jurahul wrote:

Couple of points:
1. Note that there is some precedent of describing CPP names or types in MLIR TableGen. 
2. Adding a new bitwidth to an existing target should be a rare event, so this should affect a relatively small number of developers.
3. This is an opt-in, so by default folks will get the current behavior.

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