[X86] Always consider inserting a vXi1 vector into the lsbs of a zero vector...

[X86] Always consider inserting a vXi1 vector into the lsbs of a zero vector to be legal during lowering. Add isel patterns to emit shifts.

Previously we only allowed these through if the subvector came from a compare or test instruction which we would again check for during isel.

With this change we only check for the compare and test instructions during isel and have fallback patterns that emit the shifts if needed.

I noticed that in a lot of cases we don't actually see the compare during lowering and rely on an odd legalization of concat_vectors with a zero vector as the second argument. This keeps the concat_vectors around long enough for a later dag combine to expose the compare then we re-legalize the concat_vectors and catch the compare.

llvm-svn: 320134