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

[Craig Topper via llvm-commits]

topperc wrote:

What if we duplicated decodeMCInst and decodeInstruction based on bitwidth but left them as template functions. That would remove the unused switch cases in each decodeMCInst. 
Unfortunately, we'd have to update targets to call the correct decodeInstruction with the bitwidth in the name.

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