[llvm] Change `half` to use soft promotion rather than `PromoteFloat` (PR #152727)

[Trevor Gross via llvm-commits]

tgross35 wrote:

https://github.com/llvm/llvm-project/pull/152723 includes the basline test, which should land first.

@rorth or @koachan, could you review this?

https://github.com/llvm/llvm-project/pull/152727