[X86] Add a special case to LowerCONCAT_VECTORSvXi1 to handle concatenating...

[X86] Add a special case to LowerCONCAT_VECTORSvXi1 to handle concatenating zero vectors followed by one non-zero vector followed by undef vectors.

For such a case we should only need a KSHIFTL, but we were
previously generating a KSHIFTL followed by a KSHIFTR because
we mistakenly believed we need to zero the undef elements.

llvm-svn: 369224