glsl: Calcluate Mesa state slots in front-end instead of back-end

glsl: Calcluate Mesa state slots in front-end instead of back-end

This should be the last bit of infrastructure changes before
generating GLSL IR for assembly shaders.

This commit leaves some odd code formatting in ir_to_mesa and brw_fs.
This was done to minimize whitespace changes / reindentation in some
loops.  The following commit will restore formatting sanity.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Chad Versace <chad.versace@intel.com>