st/mesa: skip lower_output_reads when possible

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_glsl_to_tgsi.cpp

@@ -6445,7 +6445,8 @@ get_mesa_program_tgsi(struct gl_context *ctx,
                                                prog->Parameters);
 
    /* Remove reads from output registers. */
-   lower_output_reads(shader->Stage, shader->ir);
+   if (!pscreen->get_param(pscreen, PIPE_CAP_TGSI_CAN_READ_OUTPUTS))
+      lower_output_reads(shader->Stage, shader->ir);
 
    /* Emit intermediate IR for main(). */
    visit_exec_list(shader->ir, v);