GSA: Fix iterator invalidation when expanding gamma trees

[zeinabPourgheisari]

While iterating over gatesPerBlock inside convertPhiToGamma function, the gamma expansion logic may insert new
entries into the same DenseMap. This can invalidate iterators and lead to
crashes in specific CFG/GSA patterns. The iteration is now performed over a
stable copy so that gatesPerBlock can be safely extended during gamma
expansion.

P.S. This still needs to be tested on all integration tests; once that is
done, the PR will be marked as non-draft.

[AyaElAkhras]

Thanks for reporting this, I see the issue. Just one thing to double-check: all of the gates added by expandGammaTree are already gamma gates and therefore do not need to go through the convertPhiToGamma logic, right?

[zeinabPourgheisari]

Just one thing to double-check: all of the gates added by expandGammaTree are already gamma gates and therefore do not need to go through the convertPhiToGamma logic, right?

Yes, exactly.