RFR: 8356453: C2: assert(!vbox->is_Phi()) during vector box expansion

[Vladimir Ivanov]

Some Vector API tests fail with an assert during `PhaseVector::expand_vbox_node()`. The assert itself is the culprit, since it doesn't cover the case when VBox node is already expanded.

Proposed fix adjusts the assert.

Also, extended `PhaseVector::optimize_vector_boxes()` with `StressMacroExpansion` support.

Testing: hs-tier1 - hs-tier5

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/25110/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25110&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356453
  Stats: 11 lines in 2 files changed: 9 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25110.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25110/head:pull/25110

PR: https://git.openjdk.org/jdk/pull/25110