Cheeky performance improvement on big DAGs

[boxysean]

resolves #6697

Description

Small optimization on manifest parsing benefitting large DAGs.

Swapping the conditions of the if statement as shown in 9f96087 short-circuits the relatively expensive _already_known method call. In a test DAG provided by my client, with 8468 models and 17103 tests, this significantly reduced calls to _already_known and reduced runtime in my test environment. See profiling screenshot of a dbt compile command:

I don't know precisely what this will translate to in real-world results, but it's safe to say (1) there are fewer calls to a relatively expensive method, and (2) the logic is the same. My client reports any dbt build command takes 15 minutes startup time -- I hope afterwards the startup time will be between 11-12 minutes.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[dbeatty10]

Very smart @boxysean ! 🧠 Commutativity for the win! 🏆

One small suggestion:

• As an alternative to #6073, I created a stand-alone issue for this precision change that we could link back to instead
• We can treat #6073 more like #5527

Open to discussion if this seems unnecessary or undesirable.

[boxysean]

Perfect, plan looks good @dbeatty10! Thanks, let me know if you need anything else from me.

[dbeatty10]

Let's goooo!

I will merge once it finishes its additional round of CI checks.