RFR: JDK-8308994: C2: Re-implement experimental post loop vectorization

[Pengfei Li]

## TL;DR

This patch completely re-implements C2's experimental post loop vectorization for better stability, maintainability and performance. Compared with the original implementation, this new implementation adds a standalone loop phase in C2's ideal loop phases and can vectorize more post loops. The original implementation and all code related to multi-versioned post loops are deleted in this patch. More details about this patch can be found in the document replied in this pull request.

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Commit messages:
 - JDK-8308994: C2: Re-implement experimental post loop vectorization

Changes: https://git.openjdk.org/jdk/pull/14581/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14581&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308994
  Stats: 2533 lines in 42 files changed: 1951 ins; 521 del; 61 mod
  Patch: https://git.openjdk.org/jdk/pull/14581.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14581/head:pull/14581

PR: https://git.openjdk.org/jdk/pull/14581