Use VEC_PERM_EXPR in the vectorizer

[Dominique Dhumieres]

> Tested on x86_64-linux, --with-cpu=corei7 and ppc64-linux, --with-cpu=G5.
>
> Committed.

This may have caused http://gcc.gnu.org/ml/gcc-bugs/2011-10/msg01534.html

Dominique