[SMTChecker] Fix BMC targets with FP

test/libsolidity/smtCheckerTests/operators/fixed_point_add.sol

@@ -0,0 +1,12 @@
+pragma experimental SMTChecker;
+contract test {
+	function f() internal pure {
+		ufixed a = uint64(1) + ufixed(2);
+	}
+}
+// ----
+// Warning: (80-88): Unused local variable.
+// Warning: (91-100): Type conversion is not yet fully supported and might yield false positives.
+// Warning: (103-112): Type conversion is not yet fully supported and might yield false positives.
+// Warning: (91-112): Underflow (resulting value less than 0) happens here
+// Warning: (91-112): Overflow (resulting value larger than 2**256 - 1) happens here

test/libsolidity/smtCheckerTests/operators/fixed_point_compound_add.sol

@@ -0,0 +1,8 @@
+pragma experimental SMTChecker;
+contract C {
+  fixed[] b;
+  function f() internal { b[0] += 1; }
+}
+// ----
+// Warning: (84-93): Underflow (resulting value less than 0) happens here
+// Warning: (84-93): Overflow (resulting value larger than 2**256 - 1) happens here