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

[Derek Schuff via llvm-commits]

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

+cc @brendandahl 
I think this makes sense to match other targets for the non-fp16 case. Once we pick up the fp16 proposal again we should think more about how the ABI should work in that case.

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