-vast-snapshot-at should have an option to end after all passes are snapshotted.

[lkorenc]

Prerequisites

• I have read the documentation and the proposed feature is not implemented.

Description

We should have an option to stop the compilation after all snapshots have been taken. This is helpful for testing scenarios, where we may want to only test a few of the early passes.
Natural way to do this would be -vast-emit-mlir=snaphots (we want a separate target here to not confuse frontend about backend output) and then instrument the pipeline runner to only emit required passes.