[llvm] [WebAssembly] Legalize i128 to v16i8 for setcc, expand memcmp for 16 byte loads with simd128 (PR #149461)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM!

Just for the PR title, I would say `Combine i128 to v16i8 for setcc` since it's technically a combine, not legalization.

And make sure to flesh out the PR description with a few sentences about how ExpandMemcmp can expand larger 128 bit loads, but they're emitted as i128s and we need to combine them into v16i8 types for efficient lowering.

https://github.com/llvm/llvm-project/pull/149461