Add CI tests for MLIR dialect

[sampsyo]

Let's add a few simple tests for bril2mlir and mlir2bril as implemented in #463. To enable them in CI, we'll want to use a similar strategy to our Rust workflow: i.e., only trigger these tests when the brilir/ directory changes. Checking out and building MLIR will be super slow, so we will need to be sure that this doesn't happen on irrelevant commits.