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

[Rahul Joshi via llvm-commits]

jurahul wrote:

One option to have backends choose between the two versions (i.e., in total support 3 modes currently, templated, with a decode impl function for less code size, and with a per-bitwidth specialized decodeInstruction) but I feel that too many options and too much complexity and maintenance over time. I think we want to drill down to just one form that works well for most backends and deprecate the templated support eventually

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