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

[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)

-------------

Commit messages:
 - compile::init_start replaced with assert

Changes: https://git.openjdk.org/jdk/pull/20715/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20715&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330159
  Stats: 12 lines in 3 files changed: 0 ins; 9 del; 3 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