glsl: Expose IR builder support for arbitrary swizzling.

glsl: Expose IR builder support for arbitrary swizzling.

IR builder already offers a lot of swizzling functions, such as
swizzle_xxxx, swizzle_z, or swizzle_for_size.

The swizzle_xxxx style is convenient if you statically know which
components you want.  swizzle_for_size is great if you want to select
the first few components.  However, if you want to select components
based on, say, a loop counter, none of those are sufficient.

IR builder actually already had support for arbitrary swizzling, but
didn't expose it.  This patch exposes that API.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>