and was actually very wrong, fix it and make it simpler. Also remove the
ConcatVectors function, which is unused now.

- Fix a introduction of useless nodes in r126664 and r126264. The
VUNPCKL* should never be introduced cause we don't want duplicate
nodes for 128 AVX and non-AVX modes, the actual instruction
difference only exists during isel, but not for target specific DAG
nodes. We only introduce V* target nodes when there is no 128-bit
version already there.

- Fix a fragile test and make it more useful.

llvm-svn: 135729