-Inspected a AVX code block added by someone in early Feb. This was never used

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