[SMTChecker] Support unary inc/dec for array/mapping access

test/libsolidity/smtCheckerTests/operators/unary_sub_array.sol

@@ -0,0 +1,18 @@
+pragma experimental SMTChecker;
+
+contract C
+{
+	uint[] array;
+	function f(uint x) public {
+		array[x] = 5;
+		uint a = --array[x];
+		assert(array[x] == 4);
+		assert(a == 4);
+		uint b = array[x]--;
+		assert(array[x] == 3);
+		// Should fail.
+		assert(b > 4);
+	}
+}
+// ----
+// Warning: (240-253): Assertion violation happens here