Enable wasm_simd_post_mvp flag in fuzzer

Bitmask requires wasm_simd_post_mvp because it was merged into SIMD
proposal after 84 cut, which Origin Trial starts.

For now, bitmask is the only instruction that requires this flag to be
set, and no other post mvp instructions are included in the fuzzer.

We should revert this change (and also move bitmask out of the flag)
after this OT is over.

Bug: chromium:1098666
Change-Id: I7d45c805aaa18bfc1a5180e70b912d5f17d4a31d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2264628Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68549}

test/fuzzer/wasm-fuzzer-common.cc

@@ -277,6 +277,10 @@ void WasmExecutionFuzzer::FuzzWasmModule(Vector<const uint8_t> data,
 #undef ENABLE_STAGED_FEATURES
   // SIMD is not included in staging yet, so we enable it here for fuzzing.
   EXPERIMENTAL_FLAG_SCOPE(simd);
+  // TODO(v8:10308): Bitmask was merged into proposal after 84 cut, so it was
+  // left gated by this flag. In order to fuzz it, we need this flag. This
+  // should be removed once we move bitmask out of post mvp.
+  FLAG_SCOPE(wasm_simd_post_mvp);
 
   // Strictly enforce the input size limit. Note that setting "max_len" on the
   // fuzzer target is not enough, since different fuzzers are used and not all