RFR: 8325520: Vector loads with offsets incorrectly compiled [v4]

[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.

Damon Fenacci has updated the pull request incrementally with five additional commits since the last revision:

 - JDK-8325520: take advantage of store_Opcode to avoid more checks
 - JDK-8325520: remove check for offsets/mask equivalence if load->store or store->load
 - JDK-8325520: merge
 - JDK-8325520: fix missing semicolon
 - JDK-8325520: address PR comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18347/files
  - new: https://git.openjdk.org/jdk/pull/18347/files/b7e5fe02..d25bcacf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18347&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18347&range=02-03

  Stats: 143 lines in 3 files changed: 15 ins; 126 del; 2 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