[llvm] [AArch64] Lower bfloat FADD/SUB to BFMLAL top/bottom instructions (PR #174814)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm requested changes to this pull request.

I'm requesting changes just so the PR is not accidentally merged.

It's still not clear what the wins are here so I've created https://github.com/llvm/llvm-project/pull/184357 to improve the generated code while maintaining the existing idiom. When that lands please update this PR, at which point I think it'll be easier to understand this PR's value.

https://github.com/llvm/llvm-project/pull/174814