Refactor the traversal of operations to Convert in DialectConversion.

This cl changes the way that operations/blocks to convert are collected/traversed so that parent region operations can be legalized before their bodies. Most RewritePatterns for region operations assume that the entry arguments to each region are yet to be converted. Given that the bodies are currently converted first, this makes it difficult to fit these patterns into the same run as one converting types.

The operations/blocks to convert are now collected before any legalization has run, which simplifies the conversion logic itself, as legalization may insert new operations, move blocks, etc.

PiperOrigin-RevId: 258170158