RFR: 8308994: C2: Re-implement experimental post loop vectorization [v3]

[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.

Pengfei Li has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - Merge branch 'master' (as of Sep 20) into postloop
 - Address part of comments from Emanuel
 - JDK-8308994: C2: Re-implement experimental post loop vectorization

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Changes: https://git.openjdk.org/jdk/pull/14581/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14581&range=02
  Stats: 1999 lines in 38 files changed: 1993 ins; 2 del; 4 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