new free function tests

test/libsolidity/smtCheckerTests/file_level/enum.sol

@@ -0,0 +1,23 @@
+enum E {
+	READ,
+	WRITE
+}
+
+function allocate(bool b) pure returns (E) {
+	if (b) return E.READ;
+	return E.WRITE;
+}
+
+contract C {
+	function f() public pure {
+		E e1 = allocate(true);
+		assert(e1 == E.READ); // should hold
+		E e2 = allocate(false);
+		assert(e2 == E.READ); // should fail
+		assert(allocate(false) == E.WRITE); // should hold
+	}
+}
+// ====
+// SMTEngine: all
+// ----
+// Warning 6328: (247-267): CHC: Assertion violation happens here.\nCounterexample:\n\ne1 = 0\ne2 = 1\n\nTransaction trace:\nC.constructor()\nC.f()\n    allocate(true) -- internal call\n    allocate(false) -- internal call