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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369224 91177308-0d34-0410-b5e6-96231b3b80d8