glsl: build stageref mask using IR, not symbol table

glsl: build stageref mask using IR, not symbol table

Instead of using symbol table, build mask by inspecting IR. This
change is required by further patches to move resource list creation
to happen later when symbol table does not exist anymore.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Martin Peres <martin.peres@linux.intel.com>