[llvm] [Lanai] Use `softPromoteHalfType` (PR #175137)

[Trevor Gross via llvm-commits]

tgross35 wrote:

Alright. I figure you'll wind up reviewing so these should be all set:

* This one
* https://github.com/llvm/llvm-project/pull/175138
* https://github.com/llvm/llvm-project/pull/175139
* https://github.com/llvm/llvm-project/pull/175140
* https://github.com/llvm/llvm-project/pull/175141

https://github.com/llvm/llvm-project/pull/175137