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

[Emanuel Peter]

Subtask of https://github.com/openjdk/jdk/pull/16620

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:
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`.

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Commit messages:
 - _vtrace is moved to VLoop
 - comment update
 - cosmetics
 - rename in preconditions
 - remove loop_transform_helper
 - fix small bug
 - preallocate memory
 - more refactoring
 - moved mark_reductions
 - rm init, and revert some other stuff
 - ... and 1 more: https://git.openjdk.org/jdk/compare/fe0eec7e...06d83797

Changes: https://git.openjdk.org/jdk/pull/17624/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17624&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324890
  Stats: 625 lines in 9 files changed: 255 ins; 199 del; 171 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