[SMTChecker] Refactor VariableUsage

test/libsolidity/smtCheckerTests/modifiers/modifier_inside_branch_assignment.sol

@@ -0,0 +1,23 @@
+pragma experimental SMTChecker;
+
+contract C {
+	uint x;
+	address owner;
+
+	modifier onlyOwner {
+		if (msg.sender == owner) _;
+	}
+
+	function f() public onlyOwner {
+		x = 0;
+	}
+	function g(uint y) public {
+		x = 1;
+		if (y > 0)
+			f();
+		// Fails for {y = >0, msg.sender == owner, x = 0}.
+		assert(x > 0);
+	}
+}
+// ----
+// Warning: (287-300): Assertion violation happens here