RFR: 8333099: Missing check for is_LoadVector in StoreNode::Identity

[Damon Fenacci]

[JDK-8325520](https://bugs.openjdk.org/browse/JDK-8325520) introduced a check for type equality in `StoreNode::Identity` in the specific case of a load vector followed by a store vector.
Unfortunately the memory node operand might actually not be of type `LoadVector`. So, before retrieving its type, a check for `is_LoadVector` is necessary.

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Commit messages:
 - JDK-833099: missing check for is_LoadVector in StoreNode::Identity

Changes: https://git.openjdk.org/jdk/pull/19449/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19449&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333099
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19449.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19449/head:pull/19449

PR: https://git.openjdk.org/jdk/pull/19449