[wasm-simd] Add a flag to allow SSSE3 SIMD codegen

Bug: v8:11154
Change-Id: Idc1ae98c2f4c0b2924f34c76b56f230cb091c16e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2600360Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71855}

src/codegen/x64/assembler-x64-inl.h

@@ -17,7 +17,11 @@ namespace internal {
 
 bool CpuFeatures::SupportsOptimizer() { return true; }
 
-bool CpuFeatures::SupportsWasmSimd128() { return IsSupported(SSE4_1); }
+bool CpuFeatures::SupportsWasmSimd128() {
+  if (IsSupported(SSE4_1)) return true;
+  if (FLAG_wasm_simd_ssse3_codegen) return true;
+  return false;
+}
 
 // -----------------------------------------------------------------------------
 // Implementation of Assembler

src/flags/flag-definitions.h

@@ -907,6 +907,7 @@ DEFINE_BOOL(wasm_grow_shared_memory, true,
 DEFINE_BOOL(wasm_simd_post_mvp, false,
             "allow experimental SIMD operations for prototyping that are not "
             "included in the current proposal")
+DEFINE_BOOL(wasm_simd_ssse3_codegen, false, "allow wasm SIMD SSSE3 codegen")
 DEFINE_IMPLICATION(wasm_simd_post_mvp, experimental_wasm_simd)
 
 DEFINE_BOOL(wasm_code_gc, true, "enable garbage collection of wasm code")