Replace BTreeMap with IndexMap for stored ScheduleBuildPasses

[ItsDoot]

Objective

• Part of Extensible Schedule Graphs #20115

We currently store schedule build passes as a BTreeMap<TypeId, _>; however TypeId doesn't have a useful ordering for us, and is subject to change across compiles.

Solution

Switch to a IndexMap<TypeId, _>, so we can iterate in insertion order instead.

Testing

Added a test to ensure consistent iteration order.

[hymm]

I agree that insertion order makes more sense here. If this becomes a more popular feature we'll probably have to add a way for the user to order the passes.