RFR: 8331764: C2 SuperWord: refactor _align_to_ref/_mem_ref_for_main_loop_alignment

[Emanuel Peter]

This PR accomplishes these things:
- Rename `_align_to_ref` -> `_mem_ref_for_main_loop_alignment`.
- Move the `mem_ref` finding for alignment out of `SuperWord::find_adjacent_refs`. This is too early, and we don't even know if the relevant `mem_ref` is going to be vectorized. It makes more sense to pick a `mem_ref` directly in `SuperWord::adjust_pre_loop_limit_to_align_main_loop_vectors`, where we already know what packs are going to be vectorized.
- For the alignment width (aw), we can use the `vector_width` of the pack to which the `mem_ref` belongs, rather than the potentially much larger `vector_width_in_bytes`. I track this with `_aw_for_main_loop_alignment` now.

I need this for https://github.com/openjdk/jdk/pull/18822, and decided to split it out into an independent change.

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Commit messages:
 - 8331764

Changes: https://git.openjdk.org/jdk/pull/19115/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19115&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331764
  Stats: 67 lines in 2 files changed: 41 ins; 20 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/19115.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19115/head:pull/19115

PR: https://git.openjdk.org/jdk/pull/19115