[TESTCASE] AltiVec code uses wrong rtl expression

[Aldy Hernandez]

> They shouldn't be identical
>
> vmrghh = vector merge high
> vmrglh = vector merge low
>
> They should be selecting from opposite ends of the input.

daniel (berlin), could you please fix these?  these are all from your
patches, and it seems all the vector merge instructions suffer from the
same anomaly.

(i don't really understand the vector select patterns)-- plus, i'm
being lazy.

thank you.

aldy