Fuzzer: Run --dce when GC is enabled (WebAssembly#5677)

DCE at the end avoids issues with non-nullable local operations in unreachable
code, which is still being discussed. This PR avoids fuzzer errors for now, but we
should revert it when we have a proper fix.

See

* WebAssembly#5599
* WebAssembly#5665
* WebAssembly/function-references#98

scripts/fuzz_opt.py

@@ -181,6 +181,15 @@ def randomize_fuzz_settings():
         FUZZ_OPTS += ['--legalize-js-interface']
     else:
         LEGALIZE = False
+
+    # if GC is enabled then run --dce at the very end, to ensure that our
+    # binaries validate in other VMs, due to how non-nullable local validation
+    # and unreachable code interact. see
+    #   https://github.com/WebAssembly/binaryen/pull/5665
+    #   https://github.com/WebAssembly/binaryen/issues/5599
+    if '--disable-gc' not in FEATURE_OPTS:
+        FUZZ_OPTS += ['--dce']
+
     print('randomized settings (NaNs, OOB, legalize):', NANS, OOB, LEGALIZE)