New tests.

test/libsolidity/semanticTests/arithmetics/unchecked_called_by_checked.sol

@@ -0,0 +1,17 @@
+contract C {
+    function add(uint16 a, uint16 b) public returns (uint16) {
+        unchecked {
+            return a + b;
+        }
+    }
+
+    function f(uint16 a) public returns (uint16) {
+        return add(a, 0x100) + 0x100;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f(uint16): 7 -> 0x0207
+// f(uint16): 0xffff -> 511
+// f(uint16): 0xfeff -> FAILURE