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

[Vladimir Kozlov]

On Thu, 8 May 2025 01:48:17 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> 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

Looks good.

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Marked as reviewed by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25110#pullrequestreview-2825531981