glsl/linker: Optimize swizzles again after linking

glsl/linker: Optimize swizzles again after linking

Without this, the SPIR-V generator has to deal with a bunch of junk
like:

    (swiz z (swiz xxx (swiz x (var_ref packed:binormal.z,light_dir))))

It seems better to cull that stuff out than to add code to deal with
it.  The problem is the way swizzles to and from scalars have to be
handled in SPIR-V.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>