nir: add ACCESS_KEEP_SCALAR, preventing vectorization

The comment explains the reason.

Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30208>

src/compiler/nir/nir_opt_load_store_vectorize.c

@@ -1066,6 +1066,9 @@ is_strided_vector(const struct glsl_type *type)
 static bool
 can_vectorize(struct vectorize_ctx *ctx, struct entry *first, struct entry *second)
 {
+   if ((first->access | second->access) & ACCESS_KEEP_SCALAR)
+      return false;
+
    if (!(get_variable_mode(first) & ctx->options->modes) ||
        !(get_variable_mode(second) & ctx->options->modes))
       return false;

src/compiler/shader_enums.h

@@ -1145,6 +1145,19 @@ enum gl_access_qualifier
     * on AGX, used for some internal shaders.
     */
    ACCESS_IN_BOUNDS_AGX = (1 << 14),
+
+   /**
+    * Disallow vectorization.
+    *
+    * On some hw (AMD), sparse buffer loads return 0 for all components if
+    * a sparse load starts on a non-resident page, crosses the page boundary,
+    * and ends on a resident page. Sometimes we want it to return 0 only for
+    * the portion of the load that's non-resident, and load values for
+    * the portion that's resident. The workaround is to scalarize such loads
+    * and disallow vectorization. This is used by an internal copy_buffer
+    * shader where the API wants to copy all bytes that are resident.
+    */
+   ACCESS_KEEP_SCALAR = (1 << 15),
 };
 
 /**