RFR: 8375010: C2 VectorAPI: assert(vbox->is_CheckCastPP()) failed: should be expanded

[Kerem Kat]

The check `vect->is_Vector() || vect->is_LoadVector()` doesn't handle `Proj` nodes that resolve to vector types, causing an assertion failure when such nodes flow through a `Phi` into `VectorBox`.

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Commit messages:
 - 8375010: C2 VectorAPI: assert(vbox->is_CheckCastPP()) failed: should be expanded

Changes: https://git.openjdk.org/jdk/pull/29200/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29200&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375010
  Stats: 55 lines in 2 files changed: 54 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29200.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29200/head:pull/29200

PR: https://git.openjdk.org/jdk/pull/29200