glsl: Drop bogus is_vertex_input from add_shader_variable().

glsl: Drop bogus is_vertex_input from add_shader_variable().

stage_mask is a bitmask of shader stages, so the proper comparison would
be (1 << MESA_SHADER_VERTEX), not MESA_SHADER_VERTEX itself.

But we only care for structure types, and VS inputs cannot be structs.
So we can just drop this entirely.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>