[X86] Rewrite LowerAVXCONCAT_VECTORS similar to how we handle vXi1 concats.

[X86] Rewrite LowerAVXCONCAT_VECTORS similar to how we handle vXi1 concats.

This better able to detect undef and zeros pieces in the concat. Or cases when only one subvector is non-zero. This allows us to avoid silly things like double inserts into progressively larger undefs.

This still builds 512 bit concats of 128 bits by building up through 256 bits first. But I don't know if that's best.

We probably want to merge this with the vXi1 concat code since they are very similar.

llvm-svn: 327454