glsl/linker: link shaders in the opposite order (from fragment to vertex)

glsl/linker: link shaders in the opposite order (from fragment to vertex)

This ensures that inter-shader outputs and inputs are properly eliminated
across 3 or more shader stages. The behavior is unchanged with 2 or less
shader stages.

For example, elimination of unused FS inputs causes elimination of matching
GS outputs, which causes elimination of the GS inputs that were needed for
evaluation of the eliminated GS outputs, which causes elimination of
matching VS outputs. An unused FS input is all that's needed to trigger
this chain reaction.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>