[wasm] [fuzzer] Generate sequences of kWasmStmt

We were generating sequences of instructions for generating i32, i64,
f32 and f64 values, but not for generating an instruction without a
result value. This CL adds that.

R=ahaas@chromium.org

Change-Id: I5c17d4182dfc6a827c7cdaa611ba7941b9c5d12f
Reviewed-on: https://chromium-review.googlesource.com/897790Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51058}

test/fuzzer/wasm-compile.cc

@@ -479,6 +479,7 @@ void WasmGenerator::Generate<kWasmStmt>(DataRange& data) {
   if (recursion_limit_reached() || data.size() == 0) return;
 
   constexpr generate_fn alternates[] = {
+      &WasmGenerator::sequence<kWasmStmt, kWasmStmt>,
       &WasmGenerator::block<kWasmStmt>,
       &WasmGenerator::loop<kWasmStmt>,
       &WasmGenerator::if_<kWasmStmt, kIf>,