RFR: 8272377: assert preconditions that are ensured when created in add_final_edges

[Yi Yang]

I think we can assert preconditions, which are ensured by add_node_to_connection_graph when they were first added to CG, in add_final_edges when they are delay handled, then we can skip checking these preconditions in runtime(assertions instead). This PR also does some code refactor.

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Commit messages:
 - remove whitespace
 - change

Changes: https://git.openjdk.java.net/jdk/pull/5101/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5101&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272377
  Stats: 148 lines in 2 files changed: 37 ins; 43 del; 68 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5101.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5101/head:pull/5101

PR: https://git.openjdk.java.net/jdk/pull/5101