chore: put back flatten_cfg checks

[asterite]

Description

Problem

Follow up to #9200

Summary

#9200 introduced a couple of checks but they were left commented because of a failure. It turns out that in the failing test's SSA the precondition didn't hold, so it made sense for it to fail. I slightly adjusted the test so it doesn't fail (not sure if that changes the test's purposes though).

Additional Context

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation 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.

[asterite]

@vezenovm Sorry, accidentally clicked on asking a review but I didn't want to

[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: 3655b28	Previous: 28abb68	Ratio
test_report_AztecProtocol_aztec-packages_noir-projects_noir-protocol-circuits_crates_blob	3 s	2 s	1.50
test_report_AztecProtocol_aztec-packages_noir-projects_noir-protocol-circuits_crates_private-kernel-lib	2 s	1 s	2

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

CC: @TomAFrench

[asterite]

Hm, it seems failures still happen in real programs...