[llvm] [PowerPC] Change `half` to use soft promotion rather than `PromoteFloat` (PR #152632)

[Trevor Gross via llvm-commits]

tgross35 wrote:

@chenzheng1030, @EsmeYi, @lei137, @RolandF77 could you review this?

Only the last commit "[PowerPC] Change half to use soft promotion" is relevant to be reviewed for this PR. There are other commits here because it needs a few other things to land first:

* https://github.com/llvm/llvm-project/pull/152662
* https://github.com/llvm/llvm-project/pull/152684
* https://github.com/llvm/llvm-project/pull/152625

I'm avoiding marking this as "ready to review" for now so it doesn't ping everybody from the backends touched in these other PRs, but the change to soft promotion here should be ready to review.

https://github.com/llvm/llvm-project/pull/152632