Relax restrictions on immutable initialization

test/libsolidity/semanticTests/immutable/delete.sol

@@ -0,0 +1,15 @@
+contract C {
+    uint8 immutable public a;
+    uint8 immutable public b = 0x42;
+    uint public c;
+
+    constructor() {
+        delete a;
+        delete b;
+        c = b * 2 + a;
+    }
+}
+// ----
+// a() -> 0
+// b() -> 0
+// c() -> 0

test/libsolidity/semanticTests/immutable/increment_decrement.sol

@@ -0,0 +1,18 @@
+contract C {
+    int immutable x = 1;
+    int immutable y = 3;
+
+    constructor() {
+        x--;
+        --x;
+        y++;
+        ++y;
+        --y;
+    }
+
+    function f() public view returns (int, int) {
+        return (x, y);
+    }
+}
+// ----
+// f() -> -1, 4

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

test/libsolidity/semanticTests/immutable/uninitialized.sol

@@ -0,0 +1,11 @@
+contract C {
+    uint immutable u;
+    bool immutable b;
+    address immutable a;
+
+    function get() public returns (uint, bool, address) {
+        return (u, b, a);
+    }
+}
+// ----
+// get() -> 0, false, 0x0