fix(ssa): Keep store whose address is used even after flattening

[vezenovm]

Description

Problem

Resolves https://github.com/noir-lang/noir/security/advisories/GHSA-pgh4-36hc-pmrx

Summary

Changes:

• Adds a defensive check that a store address is unused. This is run even when DIE is run after flattening. These cases should be prevented by our pre/post debug assertions, however, this can act as another defensive check.

Additional Context

User Documentation

Check one:

• No user documentation needed.
• Changes in docs/ included in this PR.
• [For Experimental Features] Changes in docs/ to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Test Suite Duration'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 1bb5312	Previous: 13ce5c8	Ratio
test_report_zkpassport_noir_rsa_	2 s	1 s	2

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Compilation Time'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 1bb5312	Previous: 13ce5c8	Ratio
private-kernel-inner	2.982 s	2.386 s	1.25

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Execution Time'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 1bb5312	Previous: 13ce5c8	Ratio
sha512-100-bytes	0.083 s	0.055 s	1.51

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[jfecher]

Looks like !used_values.contains(address) should be added to should_remove_store since it is now a condition on us removing the store or not

[vezenovm]

should_remove_store is a free function also used in the DIE post check so that is why I did not do this change. I could expand its definition to accept used_values though. Do you prefer that?