add example test for fixpoint iteration

[carljm]

Add a test with a simplified example case where fixpoint iteration is needed in dataflow analysis.

Inputs are Definition, Use, and Literal (to simplify the example, we exclude parsing and just take the data flow structure as input) modeling a simple language with only the following constructs:

• Literal integers and strings.
• Assignments, which are always in the form x = y + <literal> or x = <literal>.
• Branches (implicitly, because a use can have multiple reaching definitions.)
• Loops (implicitly, because a use can have a reaching definition which is the definition the use itself occurs within.)

The added tests simple and union pass; the tests cycle_converges, cycle_diverges, and multi_symbol_cycle_converges hit a Salsa cycle and fail.

The initial goal of the fixpoint iteration feature (IMO) should be to make these tests pass (not exactly as-is; presumably some cycle-handling code will need to be added in some form, but exactly what that feature should look like is what we need to determine.)

[netlify[bot]]

✅ Deploy Preview for salsa-rs canceled.

Name	Link
🔨 Latest commit	167af0b0bd91b471e336a762ec57cafb7437a4b6
🔍 Latest deploy log	https://app.netlify.com/sites/salsa-rs/deploys/67059dc4f8307300089cfcd0

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #570 will not alter performance

_{Comparing carljm:cycle-test (167af0b) with master (a20b894)}

Summary

✅ 8 untouched benchmarks