Report an error if immutables not assigned

test/libsolidity/syntaxTests/immutable/no_assignments.sol

@@ -0,0 +1,13 @@
+contract C {
+    uint immutable x;
+    constructor() {
+        x = 0;
+        while (true)
+        {}
+    }
+    function f() external view returns(uint) { return x; }
+}
+// ====
+// optimize-yul: true
+// ----
+// CodeGenerationError 1284: Some immutables were read from but never assigned, possibly because of optimization.