Fix type check for nested arrays in abi.encode/decode functions in ABIEncoderV1

- Without this fix, nested arrays are not detected as unsupported and compiler fails on an UnimplementedError.
- Now it's consistent with how structs are handled in ABIEncoderV1.

test/libsolidity/syntaxTests/specialFunctions/abi_encode_nested_dynamic_array.sol

@@ -0,0 +1,7 @@
+contract C {
+    function test() public pure {
+        abi.encode([new uint[](5), new uint[](7)]);
+    }
+}
+// ----
+// TypeError 2056: (66-96): This type cannot be encoded.