agx: Fix fragment side effects scheduling

We can't move discards across side effects, or the side effect might not happen.

Fixes KHR-GLES31.core.shader_image_load_store.basic-allFormats-load-fs
regression. Sigh.

CI is up next.

Fixes: 119e5b9719 ("agx: Schedule for register pressure")
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26056>

src/asahi/compiler/agx_pressure_schedule.c

@@ -81,6 +81,8 @@ create_dag(agx_context *ctx, agx_block *block, void *memctx)
       assert(dep != AGX_SCHEDULE_CLASS_INVALID && "invalid instruction seen");
 
       bool barrier = dep == AGX_SCHEDULE_CLASS_BARRIER;
+      bool discards =
+         I->op == AGX_OPCODE_SAMPLE_MASK || I->op == AGX_OPCODE_ZS_EMIT;
 
       if (dep == AGX_SCHEDULE_CLASS_STORE)
          add_dep(node, memory_load);
@@ -94,6 +96,10 @@ create_dag(agx_context *ctx, agx_block *block, void *memctx)
       if (dep == AGX_SCHEDULE_CLASS_COVERAGE || barrier)
          serialize(node, &coverage);
 
+      /* Make sure side effects happen before a discard */
+      if (discards)
+         add_dep(node, memory_store);
+
       if (dep == AGX_SCHEDULE_CLASS_PRELOAD)
          serialize(node, &preload);
       else