RFR: 8343747: C2: TestReplicateAtConv.java crashes with -XX:MaxVectorSize=8

[Roland Westrelin]

Crash occurs when attempting to create a `Replicate` node that's input
to a `VectorCast` node (for a `ConvL2I`) that's not supported by the
platform (when run with `MaxVectorSize=8`). I think the pack for the
`VectorCast` should be filtered out earlier as not implemented and I
propose adding a test to `VectorCastNode::implemented()` for the type
of its input to handle that corner case.

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Commit messages:
 - fix & test

Changes: https://git.openjdk.org/jdk/pull/22442/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22442&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343747
  Stats: 4 lines in 2 files changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22442.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22442/head:pull/22442

PR: https://git.openjdk.org/jdk/pull/22442