fix(ssa): fix the compile-time check for equality in acir-gen

[guipublic]

Description

Summary of changes

Check for equality before creating a witness and a constraint, in case the equality is known at compile-time.

Test additions / changes

This issue happens with the poseidonsponge_x5_254 test, but it is not part of the CI because it would be too long.

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.
• This PR requires documentation updates when merged.

[kevaundray]

This looks good to me