chore(ssa): Separate the two underconstrained checks

[aakoshh]

Description

Problem

The Ssa::check_for_underconstrained_values and Ssa::check_for_missing_brillig_constraints passes are defined in the same module, but don't share any functionality. The module is huge and it's hard to remember which method does what, they sound ambiguous.

Summary

Puts the above two in their own module.

Also added the diagram from #6658 (comment) to help understand what the check_for_missing_brillig_constraints is trying to do.

Additional Context

Hopefully this will make #11816 easier to work on, as one can ignore the "other" pass.

For example the ticket says:

The check_for_underconstrained_values pass results in a false positive flag when fed the following SSA:

But it's actually the check_for_missing_brillig_constraints that does this, as seen in #11808

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 'Execution Time'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 7cbf544	Previous: 7298451	Ratio
rollup-block-root	0.004 s	0.003 s	1.33

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

CC: @TomAFrench

[github-actions]

⚠️ Performance Alert ⚠️

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

Benchmark suite	Current: 7cbf544	Previous: 7298451	Ratio
rollup-tx-merge	0.002 s	0.001 s	2

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

CC: @TomAFrench