glsl: Add common swizzles to ir_builder.

glsl: Add common swizzles to ir_builder.

Now we can fold a bunch of our expression setup in ff_fragment_shader
into single-line, parseable commits.

v2: Make it actually work.  I wasn't setting num_components in the
    mask structure, and not setting up a mask structure is way easier.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>