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

[Rahul Joshi via llvm-commits]

jurahul wrote:

Requesting not review per-se but opinion on the user interface for this optimization. Choices proposed are:

- Command line option, as in this PR. +: non-intrusive in terms of .td files, -: need to parse it and parsing can be flaky.
- Per instruction record carries cpp type (@topperc 's suggestion): +: No command line option parsing flakiness, -: (IMO) too invasive, I see difficulty or increased complexity in adoption for auto-generated inst defs used in several downstream backends, needs additional validation for consistent across all insts of a given size.
- Option embedded as a new singleton records in the .td file: +:No command line option parsing flakiness, less intrusive than the option below (as the single def is standalone not attached to anything else), no consistency checks. -: ? 


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