[llvm] [AArch64] Switch to soft promoting half types. (PR #80576)

[Harald van Dijk via llvm-commits]

hvdijk wrote:

@david-arm I had another look at how to document and have amended in a way that easily extends to the other architectures when I create more PRs for them, does it look good to you like this?

https://github.com/llvm/llvm-project/pull/80576