RFR: 8326962: C2 SuperWord: cache VPointer

[Emanuel Peter]

This is a subtask of [JDK-8315361](https://bugs.openjdk.org/browse/JDK-8315361).

Parsing `VPointer` currently happens all over SuperWord. And often in quadratic loops, where we compare all-with-all loads/stores.

I propose to cache the `VPointer`s, then we can do a constant-time cache lookup rather than parsing the pointer subgraph every time.

There are now only a few cases where we cannot use the cached `VPointer`:
- `SuperWord::unrolling_analysis`: we have no `VLoopAnalyzer`, and so no submodules like `VLoopPointers`. We don't need to cache, since we only iterate over the loop body once, and create only a single `VPointer` per memop.
- `SuperWord::output`: when we have a `Load`, and try to bypass `StoreVector` nodes. The `StoreVector` nodes are new, and so we have no cached `VPointer` for them. This could be fixed somehow, but I don't want to deal with it now. I intend to refactor `SuperWord::output` soon, and can look into options at that point (either I bypass before we insert the vector nodes, or I remember what scalar memop the vector was created from, and then get the cached pointer this way).

This changeset is also a preparation step for [JDK-8325155](https://bugs.openjdk.org/browse/JDK-8325155). I will have a list of pointers, and sort them such that creating adjacent refs is much more efficient.

**Benchmarking SuperWord Compile Time**

I use the same benchmark from https://github.com/openjdk/jdk/pull/18532.

On master:

    C2 Compile Time:       56.816 s
         IdealLoop:            56.604 s
           AutoVectorize:      56.192 s


With this patch:

    C2 Compile Time:       49.719 s
         IdealLoop:            49.509 s
           AutoVectorize:      49.106 s


This saves us about `7 sec`, which is significant. I will have to see what it effect it has once we also apply https://github.com/openjdk/jdk/pull/18532, but I think the combined effect will be very significant.

-------------

Commit messages:
 - use caching
 - 8326962

Changes: https://git.openjdk.org/jdk/pull/18577/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18577&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326962
  Stats: 168 lines in 5 files changed: 122 ins; 13 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/18577.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18577/head:pull/18577

PR: https://git.openjdk.org/jdk/pull/18577