Fix reads checks for immutable variables.

test/libsolidity/syntaxTests/immutable/complex.sol

@@ -0,0 +1,8 @@
+contract A {
+  int immutable a;
+  constructor() { a = 5; }
+  function f() public { a += 7; }
+}
+
+// ----
+// TypeError 1581: (83-84): Cannot write to immutable here: Immutable variables can only be initialized inline or assigned directly in the constructor.