[X86] Go back to considering v64i1 as a legal type under...

[X86] Go back to considering v64i1 as a legal type under min-legal-vector-width=256. Scalarize v64i1 arguments and shuffles under min-legal-vector-width=256.

This reverts 3e1aee2b in favor
of a different approach.

Scalarizing isn't great codegen, but making the type illegal was
interfering with k constraint in inline assembly.