nir: replace lower_io_variables with a GLSL NIR flag

This stops using it in nir_lower_io_passes because all callers call it
only when it's true.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26918>

src/compiler/nir/nir.h

@@ -3564,6 +3564,14 @@ typedef enum {
     * location, and then also as centroid, at_offset, and at_sample.
     */
    nir_io_has_flexible_input_interpolation_except_flat = BITFIELD_BIT(0),
+
+   /* Options affecting the GLSL compiler are below. */
+
+   /**
+    * Lower load_deref/store_deref to load_input/store_output/etc. intrinsics.
+    * This is only affects GLSL compilation.
+    */
+   nir_io_glsl_lower_derefs = BITFIELD_BIT(16),
 } nir_io_options;
 
 /** An instruction filtering callback
@@ -4012,16 +4020,10 @@ typedef struct nir_shader_compiler_options {
    nir_lower_doubles_options lower_doubles_options;
    nir_divergence_options divergence_analysis_options;
 
-   /**
-    * Lower load_deref/store_deref of inputs and outputs into
-    * load_input/store_input intrinsics. This is used by nir_lower_io_passes.
-    */
-   bool lower_io_variables;
-
    /**
     * The masks of shader stages that support indirect indexing with
-    * load_input and store_output intrinsics. It's used when
-    * lower_io_variables is true. This is used by nir_lower_io_passes.
+    * load_input and store_output intrinsics. It's used by
+    * nir_lower_io_passes.
     */
    uint8_t support_indirect_inputs;
    uint8_t support_indirect_outputs;

src/compiler/nir/nir_lower_io.c

@@ -3190,8 +3190,7 @@ type_size_vec4(const struct glsl_type *type, bool bindless)
 void
 nir_lower_io_passes(nir_shader *nir, bool renumber_vs_inputs)
 {
-   if (!nir->options->lower_io_variables ||
-       nir->info.stage == MESA_SHADER_COMPUTE)
+   if (nir->info.stage == MESA_SHADER_COMPUTE)
       return;
 
    bool has_indirect_inputs =

src/gallium/drivers/radeonsi/si_get.c

@@ -1420,8 +1420,8 @@ void si_init_screen_get_functions(struct si_screen *sscreen)
     * when execution mode is rtz instead of rtne.
     */
    options->force_f2f16_rtz = true;
-   options->io_options = nir_io_has_flexible_input_interpolation_except_flat;
-   options->lower_io_variables = true;
+   options->io_options = nir_io_has_flexible_input_interpolation_except_flat |
+                         nir_io_glsl_lower_derefs;
    /* HW supports indirect indexing for: | Enabled in driver
     * -------------------------------------------------------
     * TCS inputs                         | Yes

src/mesa/state_tracker/st_glsl_to_nir.cpp

@@ -864,7 +864,7 @@ st_finalize_nir(struct st_context *st, struct gl_program *prog,
    /* Lower load_deref/store_deref of inputs and outputs.
     * This depends on st_nir_assign_varying_locations.
     */
-   if (nir->options->lower_io_variables) {
+   if (nir->options->io_options & nir_io_glsl_lower_derefs) {
       nir_lower_io_passes(nir, false);
       NIR_PASS(_, nir, nir_remove_dead_variables,
                  nir_var_shader_in | nir_var_shader_out, NULL);