glsl: Make add_interface_variables only consider the appropriate stage.

add_interface_variables() is supposed to add variables for the inputs of
the first shader stage linked into a program, and the outputs of the
last shader stage linked into a program.

From the ARB_program_interface_query specification:

"* PROGRAM_INPUT corresponds to the set of active input variables used by
   the first shader stage of <program>.  If <program> includes multiple
   shader stages, input variables from any shader stage other than the
   first will not be enumerated.

 * PROGRAM_OUTPUT corresponds to the set of active output variables
   (section 2.14.11) used by the last shader stage of <program>.  If
   <program> includes multiple shader stages, output variables from any
   shader stage other than the last will not be enumerated."

Previously, we used build_stageref here, which walks over all linked
shaders in the program.  This meant that internal varyings would be
visible.  We don't actually need any of build_stageref's code: we
already explicitly skip packed varyings, handle modes, and the name
comparisons just do a fuzzy string comparison of name with itself.

Fixes two tests: dEQP-GLES31.functional.program_interface_query.
program_{input,output}.referenced_by.referenced_by_vertex_fragment.

These tests have a VS and FS linked together into a single program.
Both stages have an input called "shaderInput".  But the FS input
should not be visible because it isn't the first stage.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Timothy Arceri <[email protected]>

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