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

[Emanuel Peter]

On Wed, 21 Jun 2023 08:24:19 GMT, Pengfei Li <pli at openjdk.org> wrote:

> ## 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.

src/hotspot/share/opto/vmaskloop.hpp line 85:

> 83: 
> 84:   // Some node check utilities
> 85:   bool is_loop_iv(Node* n) { return n == _iv; }

General code style comment, applies everywhere: add more `const` everywhere. To arguments, and the functions themselves, wherever possible.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14581#discussion_r1239730798