[mlir] Fixed double-free bug in SymbolUserMap

`SymbolUserMap` relied on `try_emplace` and `std::move` to relocate an entry to another key.  However, if this triggered the resizing of the `DenseMap`, the value was destroyed before it could be moved to the new storage location, leading to a dangling `users` reference to be inserted into the map. On destruction, since a new entry was created from one that was already freed, a double-free error occurred.

Fixed issue by re-fetching the iterator after the mutation of the container.

Differential Revision: https://reviews.llvm.org/D129345