brw: fix sample mask flag emission

It's also used for testing helper invocations.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: e3328dfa2f ("brw: only initialize sample mask flag if needed")
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38699>

src/intel/compiler/brw/brw_compile_fs.cpp

@@ -1372,10 +1372,14 @@ run_fs(brw_shader &s, bool allow_spilling, bool do_rep_send)
       }
 
       /* We handle discards by keeping track of the still-live pixels in f0.1.
-       * On Xe2+, we also predicate stores with this mask. Initialize it with
-       * the dispatched pixels if we use discard or (on Xe2) memory stores.
+       * On Xe2+, we also predicate stores and test helper invocations with
+       * this mask. Initialize it with the dispatched pixels if we use discard
+       * or (on Xe2) memory stores or helper invocation testing.
        */
-      if ((devinfo->ver >= 20 && nir->info.writes_memory) ||
+      if ((devinfo->ver >= 20 &&
+           (BITSET_TEST(nir->info.system_values_read,
+                        SYSTEM_VALUE_HELPER_INVOCATION) ||
+            nir->info.writes_memory)) ||
           wm_prog_data->uses_kill) {
 
          const unsigned lower_width = MIN2(s.dispatch_width, 16);