RFR: 8311691: C2: Remove legacy code related to PostLoopMultiversioning

[Pengfei Li]

As discussed in JDK-8308994, we are working on re-implementation of post loop vectorization and planning to refactor current SuperWord code. As nobody is using or maintaining the old implementation now, to make the refactoring work easier, we propose to remove the legacy code of the old implementation first.

This patch removes all code realted to `PostLoopMultiversioning` inside and outside SuperWord. After the removal, `SLP_extract()` in SuperWord should only work on main loops. So we also removed all `is_main_loop()` checks inside and added assertions instead.

Tested with hotspot::hotspot_all_no_apps, jdk tier1~3, langtools tier1 and 100k fuzzer tests on x86 and AArch64, no issue is found.

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Commit messages:
 - 8311691: C2: Remove legacy code related to PostLoopMultiversioning

Changes: https://git.openjdk.org/jdk/pull/14824/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14824&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311691
  Stats: 608 lines in 9 files changed: 7 ins; 564 del; 37 mod
  Patch: https://git.openjdk.org/jdk/pull/14824.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14824/head:pull/14824

PR: https://git.openjdk.org/jdk/pull/14824