[X86] Use 128-bit vector instructions for f32/f64->i64 conversions on 32-bit...

[X86] Use 128-bit vector instructions for f32/f64->i64 conversions on 32-bit targets with avx512dq and avx512vl instructions.

On 32-bit targets we can't use the scalar instruction so we
insert the scalar into a vector and use packed conversions.
Previously we used either v4f32->v4i64 or v4f64->v4i64 to avoid
some complexity creating target specific ISD opcodes for
v4f32->v2i64. But this causes extra vzeroupper instructions and
possibly frequency throttling on Intel CPUs.

This patch changes this to create a 128-bit vector and uses a
target specific ISD opcode if needed.