i965: Define some useful swizzle helper functions.

i965: Define some useful swizzle helper functions.

This defines helper functions implementing some common swizzle
transformations that are usually open-coded in the compiler back-end,
causing a lot of clutter.  Some optimization passes will become almost
trivial implemented in terms of these functions (e.g.
vec4_visitor::opt_reduce_swizzle()).

Reviewed-by: Matt Turner <mattst88@gmail.com>