constrained-generators: Fix ifElse dependencies

[MaximilianAlgehed]

Description

This solves some thorny issues that @TimSheard was having with generating things that have a lot of ifElse in them. It generally doesn't make sense to resolve the if scrutinee after resolving the cases - unlike in caseOn where you're inspecting variables so you end up deferring things more.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[TimSheard]

looks good to me. Good to add a comment about intended uses of new features.