st/mesa: add ir_lod case in GLSL->TGSI code to silence warning

src/mesa/state_tracker/st_glsl_to_tgsi.cpp

@@ -2774,6 +2774,9 @@ glsl_to_tgsi_visitor::visit(ir_texture *ir)
    case ir_txf_ms:
       assert(!"Unexpected ir_txf_ms opcode");
       break;
+   case ir_lod:
+      assert(!"Unexpected ir_lod opcode");
+      break;
    }
 
    if (ir->projector) {