RFR: 8325520: Vector loads with offsets incorrectly compiled

[Damon Fenacci]

# Issue
When loading multiple vectors using offsets or masks (e.g. `LongVector::fromArray(LongVector.SPECIES_256, storage, 0, offsets, 0` or `LongVector::fromArray(LongVector.SPECIES_256, storage, 0, longMask)`) there is an error in the C2 compiled code that makes different vectors be treated as equal even though they are not.

# Causes
On vector-capable platforms, vector loads with masks and offsets (for Long, Integer, Float and Double) create specific nodes in the ideal graph (i.e. `LoadVectorGather`, `LoadVectorMasked`, `LoadVectorGatherMasked`). Vector loads without mask or offsets are mapped as `LoadVector` nodes instead.
While running GVN loops, we can get to the situation in which there are multiple loads from the same address. If successive loads are deemed to be identical to the first one, they might get folded, which is what happens in the problematic examples of this issue. The check for identity happens in
https://github.com/openjdk/jdk/blob/481c866df87c693a90a1da20e131e5654b084ddd/src/hotspot/share/opto/memnode.cpp#L1253
This version of `Identity` is defined in `LoadNode` but it is also the one used by the subclasses `LoadVectorGather`, `LoadVectorMasked` and `LoadVectorGatherMasked`. Although this definition of `Identity` is enough for `LoadVector` nodes it is not sufficient for `LoadVectorGather`, `LoadVectorMasked` and `LoadVectorGatherMasked` ones, as the value being loaded also depends on the offsets and mask (different offsets and/or masks load completely different values). This is the reason why these nodes get folded even if they shouldn't.

# Solution
`LoadVectorGather`, `LoadVectorMasked` and `LoadVectorGatherMasked` need their own version of the `Identity` method, which specialize `LoadVector::Identity` by restricting the results to nodes that also have the same offsets and masks.

The same issue exists for _StoreVector_ nodes (i.e. `StoreVectorScatter`, `StoreVectorMasked` and `StoreVectorScatterMasked`). So, `Identity` has to be redefined there as well.

Regression tests for all versions of `Load/StoreVectorGather/Masked` have been added too.

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Commit messages:
 - JDK-8325520: fix load gather mask avx condition
 - JDK-8325520: fix tests for small species
 - JDK-8325520: add store tests
 - JDK-8325520: fix copyright notices
 - JDK-8325520: remove trailing whitespaces
 - JDK-8325520: use IR framework in tests
 - JDK-8325520: handle same offsets/masks in store identity
 - JDK-8325520: extend Store/LoadNode::Identity instead of overriding
 - JDK-8325520: fix test for no-vector case
 - Update src/hotspot/share/opto/vectornode.cpp
 - ... and 9 more: https://git.openjdk.org/jdk/compare/c5c866aa...e331b5c5

Changes: https://git.openjdk.org/jdk/pull/18347/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18347&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325520
  Stats: 1008 lines in 5 files changed: 1005 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18347.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18347/head:pull/18347

PR: https://git.openjdk.org/jdk/pull/18347