[WASM SIMD] Implement primitive shuffles.

src/wasm/function-body-decoder-impl.h

Index: src/wasm/function-body-decoder-impl.h
diff --git a/src/wasm/function-body-decoder-impl.h b/src/wasm/function-body-decoder-impl.h
index 27e95b21386de82569ad9d87952bdaf6849fe20d..7249cc8a0df96d4cebad31caed857c072548b656 100644
--- a/src/wasm/function-body-decoder-impl.h
+++ b/src/wasm/function-body-decoder-impl.h
@@ -322,6 +322,18 @@ struct SimdShiftOperand {
   }
 };
 
+// Operand for SIMD concatenation operations.
+template <bool checked>
+struct SimdConcatOperand {
+  uint8_t bytes;
+  unsigned length;
+
+  inline SimdConcatOperand(Decoder* decoder, const byte* pc) {
+    bytes = decoder->read_u8<checked>(pc + 2, "bytes");
+    length = 1;
+  }
+};
+
 #undef CHECKED_COND
 
 }  // namespace wasm