glsl: add texture gather changes

V2 [Chris Forbes]:
   - Add new pattern, fixup parameter reading.

V3: Rebase onto new builtins machinery

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/program/ir_to_mesa.cpp

@@ -2066,6 +2066,9 @@ ir_to_mesa_visitor::visit(ir_texture *ir)
    case ir_lod:
       assert(!"Unexpected ir_lod opcode");
       break;
+   case ir_tg4:
+      assert(!"Unexpected ir_tg4 opcode");
+      break;
    }
 
    const glsl_type *sampler_type = ir->sampler->type;