RFR: 8330159: [C2] Remove or clarify Compile::init_start [v5]

[Yagmur Eren]

> Compile::init_start method contained only an assertion. To cleanup, this method is removed and the locations where this method is called are replaced with the corresponding assertion. See issue: [JDK-8330159](https://bugs.openjdk.org/browse/JDK-8330159)

Yagmur Eren has updated the pull request incrementally with one additional commit since the last revision:

  adding NOT_DEBUG_RETURN instead of DEBUG_ONLY for Compile::verify_start

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20715/files
  - new: https://git.openjdk.org/jdk/pull/20715/files/0a05f15e..66e23d6f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20715&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20715&range=03-04

  Stats: 6 lines in 3 files changed: 1 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/20715.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20715/head:pull/20715

PR: https://git.openjdk.org/jdk/pull/20715