[mlir][Transforms][NFC] Turn unresolved materializations into `IRRewrite`s (#81761)

This commit is a refactoring of the dialect conversion. The dialect
conversion maintains a list of "IR rewrites" that can be committed (upon
success) or rolled back (upon failure).

This commit turns the creation of unresolved materializations
(`unrealized_conversion_cast`) into `IRRewrite` objects. After this
commit, all steps in `applyRewrites` and `discardRewrites` are calls to
`IRRewrite::commit` and `IRRewrite::rollback`.