[X86][FP16] Combine two steps conversions into direct conversion

When both v8i64 and v4f16 are not legal in a v8i64->v8f16 conversion, legalizer will breaks it into v8i64->v4i64->v4f32->v8f32->v8f16.

Given we support v4i64->v8f16, we can combine them with a shuffle instruction.

Reviewed By: LuoYuanke

Differential Revision: https://reviews.llvm.org/D143872