[DispatchCreation] Propagate tensor encodings outside dispatch regions (#24966)

Encoding propagation is currently tied to dispatch hoisting, which
prevents layout assignment from using the same rewrite rules on the
surrounding tensor graph.

Separate where propagation may happen from the rewrites themselves, and
add the function pass
`iree-dispatch-creation-propagate-data-tiling-encodings` for propagation
outside dispatches. Hoisting keeps its existing behavior. The new pass
stops at existing dispatches and block boundaries, and constant CSE
stays disabled because Flow regions do not materialize constants
themselves.

- `iree-dispatch-creation-propagate-encodings` runs in the dispatch-time
data-tiling pipeline and only swaps `set_encoding` with `tensor.cast`
producers, so adding these rewrites to it would change that pipeline.
They stay separate until bubbling through generic ops goes through the
encoding propagation interfaces (TODO(#20179)).
- Bubbling now stops at `set_encoding` ops whose encoding is not an
`EncodingAttr`, such as `#iree_encoding.padding`, instead of crashing.
`iree-dispatch-creation-hoist-encoding-ops` could already hit this
within dispatches; both passes have a regression test.

No pipeline runs the new pass yet. #24969 adds it to the experimental
early data-tiling route.

This is one of five PRs adding an experimental CPU path that
materializes data-tiling layouts before dispatch creation, so that
`linalg.mmt4d` and data-tiled convolutions fuse with their elementwise
epilogues and result unpacks.

Assisted by AI.