[MLIR][memref] Fix findDealloc() to handle > 1 dealloc for the given alloc.

- Change findDealloc() to return Optional<Operation *> and return None if > 1
  dealloc is associated with the given alloc.
- Add findDeallocs() to return all deallocs associated with the given alloc.
- Fix current uses of findDealloc() to bail out if > 1 dealloc is found.

Differential Revision: https://reviews.llvm.org/D106456