RFR: 8324890: C2 SuperWord: refactor out VLoop, make unrolling_analysis static, remove init/reset mechanism [v4]

[Emanuel Peter]

> Subtask of https://github.com/openjdk/jdk/pull/16620
> (The basic goal is to break SuperWord into different modules. This makes the code more maintainable and extensible. And eventually this allows some modules to be reused by other/new vectorizers.)
> 
> 1. Move out the shared code between `SuperWord::SLP_extract` (where we do vectorization) and `SuperWord::unrolling_analysis`, and move it to a new class `VLoop`. This allows us to decouple `unrolling_analysis` from the SuperWord object, and we can make it static.
> 2. So far, SuperWord was reused for all loops in a compilation, and then "reset" (with `SuperWord::init`) for every loop. This is a bit of a nasty pattern. I now make a new `VLoop` and a new `SuperWord` object per loop.
> 3. Since we now make more `SuperWord` objects, we allocate the internal data structures more often. Therefore, I now pre-allocate/reserve sufficient space on initialization.
> 
> Side-note about https://github.com/openjdk/jdk/pull/17604 (integrated, no need to read any more):
> I would like to remove the use of `SuperWord::is_marked_reduction` from `SuperWord::unrolling_analysis`. For starters: it is not clear what it was ever good for. Second: it requires us to do reduction marking/analysis before `unrolling_analysis`, and hence makes the reduction marking shared between `unrolling_analysis` and vectorization. I could move the reduction marking to `VLoop` now. But the `_loop_reducitons` set would have to be put on an arena, and I would like to avoid creating an arena for the `unrolling_analysis`. Plus, it would just be nicer code, to have reduction analysis together with body analysis, type analysis, etc. and all of them in only in `SLP_extract`.

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  timing code from JDK-8325159

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17624/files
  - new: https://git.openjdk.org/jdk/pull/17624/files/d04ff51b..a0721c5b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17624&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17624&range=02-03

  Stats: 3 lines in 3 files changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17624.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17624/head:pull/17624

PR: https://git.openjdk.org/jdk/pull/17624