[llvm] expandFMINIMUMNUM_FMAXIMUMNUM: Improve compare between zeros (PR #140193)

[YunQiang Su via llvm-commits]

wzssyqa wrote:

@nikic any idea about how to sync the FMAXIMUMNUM and FMAXIMUM?
I prefer use FP_ROUND for FMAXIMUMNUM first and then sync to FMAXIMUM.

FP_ROUND seems performing better since it doesn't need stack operations.

https://github.com/llvm/llvm-project/pull/140193