Added few tests.

test/libsolidity/semanticTests/operators/custom/multiple_custom_operators_same_contract.sol

@@ -0,0 +1,16 @@
+type MyInt is int;
+using {add as +} for MyInt;
+
+function add(MyInt, MyInt) pure returns (bool) {
+    return true;
+}
+
+contract C {
+    function f() public pure returns (bool t) {
+        t = MyInt.wrap(2) + MyInt.wrap(7);
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> true