[X86][FP16] Promote FP16->[U]INT to FP16->FP32->[U]INT

This is to avoid f16->i64 being lowered to `__fixhfdi/__fixunshfdi` on 32-bits since neither libgcc nor compiler-rt provide them. https://godbolt.org/z/cjWEsea5v

It also helps to improve the performance by promoting the vector type.

Reviewed By: LuoYuanke

Differential Revision: https://reviews.llvm.org/D131828