Allow reading of immutables during construction time

test/libsolidity/semanticTests/immutable/assign_from_immutables.sol

@@ -0,0 +1,20 @@
+contract C {
+	uint immutable public a;
+	uint immutable public b;
+	uint immutable public c;
+	uint immutable public d;
+
+	constructor() {
+		a = 1;
+		b = a;
+		c = b;
+		d = c;
+	}
+}
+// ====
+// compileViaYul: also
+// ----
+// a() -> 1
+// b() -> 1
+// c() -> 1
+// d() -> 1

test/libsolidity/semanticTests/immutable/fun_read_in_ctor.sol

@@ -0,0 +1,22 @@
+contract A {
+	uint8 immutable a;
+	uint8 x;
+
+	constructor() {
+		a = 3;
+		x = readA();
+	}
+
+	function readX() public view returns (uint8) {
+		return x;
+	}
+
+	function readA() public view returns (uint8) {
+		return a;
+	}
+}
+// ====
+// compileViaYul: also
+// ----
+// readX() -> 3
+// readA() -> 3

test/libsolidity/semanticTests/immutable/read_in_ctor.sol

@@ -0,0 +1,17 @@
+contract A {
+	uint8 immutable a;
+	uint8 x;
+
+	constructor() {
+		a = 3;
+		x = a;
+	}
+
+	function readX() public view returns (uint8) {
+		return x;
+	}
+}
+// ====
+// compileViaYul: also
+// ----
+// readX() -> 3