[X86][AVX512] Add support for variable ASHR v2i64/v4i64 support without VLX

Use v8i64 variable ASHR instructions if we don't have VLX.

This is a reduced version of D28537 that just adds support for variable shifts - I'll continue with that patch (for just constant/uniform shifts) once I've fixed the type legalization issue in avx512-cvt.ll.

Differential Revision: https://reviews.llvm.org/D28604

llvm-svn: 291901