[SMTChecker] Fix SMT logic error when doing compound assignment with string literlas.

test/libsolidity/smtCheckerTests/operators/compound_bitwise_string_literal.sol

@@ -0,0 +1,17 @@
+pragma experimental SMTChecker;
+
+contract C {
+    function f() public pure {
+        bytes memory y = "def";
+        y[0] &= "d";
+        assert(y[0] == "d");
+
+        y[0] |= "e";
+        assert(y[0] == "d"); // fails
+
+        y[0] ^= "f";
+        assert(y[0] == (byte("d") | byte("e")) ^ byte("f"));
+    }
+}
+// ----
+// Warning 6328: (189-208): CHC: Assertion violation happens here.

test/libsolidity/smtCheckerTests/operators/compound_bitwise_string_literal_2.sol

@@ -0,0 +1,17 @@
+pragma experimental SMTChecker;
+
+contract C {
+    function f() public pure {
+        bytes3 y = "def";
+        y &= "def";
+        assert(y == "def");
+
+        y |= "dee";
+        assert(y == "def"); // fails
+
+        y ^= "fed";
+        assert(y == (bytes3("def") | bytes3("dee")) ^ bytes3("fed"));
+    }
+}
+// ----
+// Warning 6328: (180-198): CHC: Assertion violation happens here.

test/libsolidity/smtCheckerTests/operators/compound_bitwise_string_literal_3.sol

@@ -0,0 +1,21 @@
+pragma experimental SMTChecker;
+
+contract C {
+    function f() public pure {
+        bytes32 y = "abcdefghabcdefghabcdefghabcdefgh";
+        bytes32 z = y;
+        y &= "bcdefghabcdefghabcdefghabcdefgha";
+        z &= "bcdefghabcdefghabcdefghabcdefgha";
+        assert(y == "abcdefghabcdefghabcdefghabcdefgh"); // fails
+
+        y |= "cdefghabcdefghabcdefghabcdefghab";
+        z |= "cdefghabcdefghabcdefghabcdefghab";
+        assert(y == "abcdefghabcdefghabcdefghabcd"); // fails
+
+        y ^= "abcdefghabcdefghabcdefghabcdefgh";
+        assert(y == z ^ "abcdefghabcdefghabcdefghabcdefgh");
+    }
+}
+// ----
+// Warning 6328: (262-309): CHC: Assertion violation happens here.
+// Warning 6328: (427-470): CHC: Assertion violation happens here.