glsl/linker: Always invalidate shader ins/outs, even in corner cases.

glsl/linker: Always invalidate shader ins/outs, even in corner cases.

Previously, link_invalidate_variable_locations() was only called
during assign_attribute_or_color_locations() and
assign_varying_locations().  This meant that in the corner case when
there was only a vertex shader, and varyings were being captured by
transform feedback, link_invalidate_variable_locations() wasn't being
called for the varyings.

This patch migrates the calls to link_invalidate_variable_locations()
to link_shaders(), so that they will be called in all circumstances.
In addition, it modifies the call semantics so that
link_invalidate_variable_locations() need only be called once per
shader stage (rather than once for inputs and once for outputs).

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>