Bug fixes for CCP

Two fixes to CCP

1. Delay deletion of branches with a constant condition so all unreachable nodes (including these conditions and their projections) are deleted together to avoid leaving uses of deleted nodes.
2. Compute reachability of data nodes as the join of the reachability of its uses so that data nodes in unreachable parts of code are marked unreachable.

juno_samples/nested_ccp/src/main.rs

+#![feature(future_join)]
+
+extern crate async_std;
+extern crate juno_build;
+extern crate hercules_rt;
+
+juno_build::juno!("nested_ccp");
+
+fn main() {
+    async_std::task::block_on(async {
+        let output = unsafe {
+            ccp_example().await
+        };
+        println!("{}", output);
+        assert_eq!(output, 1.0);
+    });
+}
+
+#[test]
+fn nested_ccp() {
+    main();
+}