[X86] Use CONCAT_VECTORS instead of INSERT_SUBVECTOR for padding v4i1/v2i1...

[X86] Use CONCAT_VECTORS instead of INSERT_SUBVECTOR for padding v4i1/v2i1 vector to v8i1 pre-legalize.

The CONCAT_VECTORS will be lowered to INSERT_SUBVECTOR later. In the modified cases this seems to be enough to trick a later DAG combine into running in a different order than allows the ANDs to be removed.

I'll admit this is a bit of a hack that happens to work, but using CONCAT_VECTORS is more consistent with other legalization code anyway.

llvm-svn: 321611