Fix reads checks for immutable variables.

test/libsolidity/syntaxTests/immutable/delete_and_initialize.sol

@@ -0,0 +1,9 @@
+contract C {
+    uint immutable x = 3;
+    constructor() {
+        delete x;
+    }
+}
+// ----
+// TypeError 1574: (74-75): Immutable state variable already initialized.
+// TypeError 7733: (74-75): Immutable variables cannot be read during contract creation time, which means they cannot be read in the constructor or any function or modifier called from it.