[llvm] [AMDGPU] Use v_pk_add_bf16 for scalar bf16 fadd on gfx1250/gfx13 (PR #209128)

[Harrison Hao via llvm-commits]

harrisonGPU wrote:

> This should be performed in the legalizer by introducing a vector op with a poison high half. I don't know why V_MAX_BF16_PSEUDO is here, but this should not follow that example

Yes, I referenced `V_MAX_BF16_PSEUDO`. I've now implemented it in the legalizer and used `ISD::SCALAR_TO_VECTOR` so the high lane is poison. By the way, should I also refactor `V_MAX_BF16_PSEUDO` to do the same?

https://github.com/llvm/llvm-project/pull/209128