[llvm] [InstCombine] Declare command line options in TableGen (PR #227948)

[Alexis Engelke via llvm-commits]

aengelke wrote:

> A general question I have is why the mapping to struct members does not follow the LLVM coding standard. That is, why does instcombine-max-num-phis map to max_num_phis rather than MaxNumPhis?

This was discussed in https://github.com/llvm/llvm-project/pull/226087#discussion_r4112204165. The idea was to generate the struct member name automatically from the option name for enforced consistency and reduced redundancy, but this is not possible with PascalCase (how to map "thinlto" to "ThinLTO" or vice versa?).

https://github.com/llvm/llvm-project/pull/227948