glsl: add variable mode check to build_stageref

glsl: add variable mode check to build_stageref

Currently stage reference mask is built using the variable name
only. However it can happen that input of one stage has same name
as output from another stage. Adding check of variable mode makes
sure we do not pick wrong variable.

Fixes some subcases from
   ES31-CTS.program_interface_query.no-locations

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Timothy Arceri <t_arceri@yahoo.com.au>