Relax restrictions on immutable initialization

test/libsolidity/semanticTests/immutable/multiple_initializations.sol

@@ -0,0 +1,27 @@
+contract A {
+    uint immutable x = x + 1;
+    uint immutable y = x += 2;
+
+    constructor(uint) m(x += 16) m(x += 32) {
+        x += 64;
+        x += 128;
+    }
+
+    modifier m(uint) {
+        _;
+    }
+
+    function get() public returns (uint) {
+        return x;
+    }
+}
+
+contract B is A(A.x += 8) {
+    constructor(uint) {}
+}
+
+contract C is B {
+    constructor() B(x += 4) {}
+}
+// ----
+// get() -> 0xff