Fix nested loop unroll exit value repair

[cervoliu]

Summary

• Refresh loop unroll user and phi-predecessor snapshots per loop so outer-loop exit repair sees phis introduced while unrolling inner loops.
• Add a regression test for the nested loop exit phi case from likely loop unroling bug #1264.

Fixes #1264.

Testing

• ninja check

Result: 1428 passed, 1 expected failure.

[Copilot]

Pull request overview

Fixes a nested-loop unrolling correctness bug where outer-loop exit repair could miss PHIs introduced during inner-loop unrolling, and adds a regression test covering the reported miscompile from #1264.

Changes:

• Refresh getUsers() and getPhiPredecessors() snapshots per loop during Function::unroll() so outer-loop exit repair sees PHIs introduced by inner-loop unrolling.
• Add a new Alive-TV regression test for the nested loop exit-PHI case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
ir/function.cpp	Recomputes user/phi-predecessor snapshots per loop during unrolling to fix nested-loop exit value repair.
tests/alive-tv/loops/nested-exit-phi.srctgt.ll	Adds a regression test reproducing the nested-loop exit-PHI mismatch and asserting the correct target result.

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