glsl: split out varying and uniform linking code

glsl: split out varying and uniform linking code

Here a new function link_varyings_and_uniforms() is created this
should help make it easier to follow the code in link_shader()
which was getting very large.

Note the end of the new function contains a for loop with some
lowering calls that currently don't seem related to varyings or
uniforms but they are a dependancy for converting to NIR ealier
so we move things here now to keep things easy to follow.

Reviewed-by: Eric Anholt <eric@anholt.net>