v3d: add debug assert

While lowering vpm outputs we look for the NIR variables matching
particular store output instructions and we expect to find a match,
so assert on that.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>

src/broadcom/compiler/v3d_nir_lower_io.c

                 }
                 var = scan_var;
         }
+        assert(var);
 
         /* Save off the components of the position for the setup of VPM inputs
          * read by fixed function HW.