RFR: 8272377: assert preconditions that are ensured when created in add_final_edges [v2]

[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.

Yi Yang has updated the pull request incrementally with one additional commit since the last revision:

  use Unexpected node type message

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5101/files
  - new: https://git.openjdk.java.net/jdk/pull/5101/files/77734564..bc773c78

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5101&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5101&range=00-01

  Stats: 12 lines in 1 file changed: 0 ins; 4 del; 8 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