[mlir][transform] Use TrackingListener-aware iterator for getPayloadOps

Instead of returning an `ArrayRef<Operation *>`, return at iterator that skips ops that were erased/replaced while iterating over the payload ops.

This fixes an issue in conjuction with TrackingListener, where a tracked op was erased during the iteration. Elements may not be removed from an array while iterating over it; this invalidates the iterator.

When ops are erased/removed via `replacePayloadOp`, they are not immediately removed from the mappings data structure. Instead, they are set to `nullptr`. `nullptr`s are not enumerated by `getPayloadOps`. At the end of each transformation, `nullptr`s are removed from the mapping data structure.

Differential Revision: https://reviews.llvm.org/D149847