[X86][AVX] LowerBUILD_VECTORAsVariablePermute - add support for VPERMILPV to v2i64/v2f64

Minor refactor to make it possible for LowerBUILD_VECTORAsVariablePermute to be used with a wider variety of shuffles op and types.

I'd have liked to add v4i32/v4f32 support as well but we don't see v4i32 index extractions at the moment (which is why I created D42308)

After this I intend to begin adding scaling support for PSHUFB (v8i16, v4i32, v2i64)) and VPERMPS (v4f64, v4i64).

Differential Revision: https://reviews.llvm.org/D42431

llvm-svn: 323260