nir: add nir_clear_divergence_info, use it in nir_opt_varyings

nir_opt_varyings computes vertex divergence, which isn't exactly expected
by any other passes.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31968>

src/compiler/nir/nir.h

@@ -6715,6 +6715,7 @@ void nir_divergence_analysis_impl(nir_function_impl *impl, nir_divergence_option
 void nir_divergence_analysis(nir_shader *shader);
 void nir_vertex_divergence_analysis(nir_shader *shader);
 bool nir_has_divergent_loop(nir_shader *shader);
+void nir_clear_divergence_info(nir_shader *nir);
 
 void
 nir_rewrite_uses_to_load_reg(struct nir_builder *b, nir_def *old,

src/compiler/nir/nir_divergence_analysis.c

@@ -1469,3 +1469,23 @@ nir_has_divergent_loop(nir_shader *shader)
 
    return false;
 }
+
+/* Recommended when computing divergence information in shared code such
+ * as the GLSL linker.
+ */
+void
+nir_clear_divergence_info(nir_shader *nir)
+{
+   nir_foreach_function_impl(impl, nir) {
+      nir_foreach_block(block, impl) {
+         /* true is the safer value. */
+         block->divergent = true;
+
+         nir_foreach_instr(instr, block) {
+            nir_def *def = nir_instr_def(instr);
+            if (def)
+               def->divergent = true;
+         }
+      }
+   }
+}

src/compiler/nir/nir_opt_varyings.c

@@ -4402,5 +4402,14 @@ nir_opt_varyings(nir_shader *producer, nir_shader *consumer, bool spirv,
    if (progress & nir_progress_consumer)
       nir_validate_shader(consumer, "nir_opt_varyings");
 
+   if (consumer->info.stage == MESA_SHADER_FRAGMENT) {
+      /* We have called nir_vertex_divergence_analysis on the producer here.
+       * We need to reset the divergent field to true, otherwise it will be
+       * garbage after some other passes are run, and then we end up failing
+       * assertions in some passes because src is divergent and dst isn't.
+       */
+      nir_clear_divergence_info(producer);
+   }
+
    return progress;
 }