[X86][FP16] Do not split FP64->FP16 to FP64->FP32->FP16

Truncation from double to half is not always identical to truncating to float first and then to half. https://godbolt.org/z/56s9517hd

On the other hand, expanding to float and then to double is always identical to expanding to double directly. https://godbolt.org/z/Ye8vbYPnY

Reviewed By: RKSimon, skan

Differential Revision: https://reviews.llvm.org/D130151