[SMTChecker] Fix internal error when array.push() is used as LHS of assignment

test/libsolidity/smtCheckerTests/array_members/push_as_lhs_struct.sol

@@ -0,0 +1,15 @@
+pragma experimental SMTChecker;
+contract C {
+	struct S {
+		int[] b;
+	}
+	S s;
+	struct T {
+		S s;
+	}
+	T t;
+	function f() public {
+		s.b.push() = t.s.b.push();
+		assert(s.b[s.b.length -1] == t.s.b[t.s.b.length - 1]);
+	}
+}