[patch] Two fixes to the vpermilvar optimization

[Rafael Espíndola]

>> Using a 42 in here has well defined behaviour according to the intel manual.
>
> OK. So this is cleaning things ignored bits of the constant up so the isel matchers will fire correctly? That makes sense. Can you elaborate in the comment a bit?

No, it is clearing it up so that the created shuffle is valid. In the
above case the 42 has to be converted to a 1 since vpermilvar.pd only
uses the second bit for indexing. A shufflevector with an index of 42
would be invalid.

Cheers,
Rafael