[X86][AVX512DQ] Use packed instructions for scalar FP<->i64 conversions on 32-bit targets

As i64 types are not legal on 32-bit targets, insert these into a suitable zero vector and use the packed vXi64<->FP conversion instructions instead.

Fixes PR3163.

Differential Revision: https://reviews.llvm.org/D43441

llvm-svn: 332498