[llvm] [WebAssembly] Change `half` to use soft promotion rather than `PromoteFloat` (PR #152833)

[Nikita Popov via llvm-commits]

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

LGTM

It's worth noting that this changes the ABI for passing half floats (without `+fp16`), but in a way that doesn't get exposed to Clang, which currently forces these to use i16. (This change will allow us to remove that hack.)

https://github.com/llvm/llvm-project/pull/152833