[mlir][TilingInterface] Fix `iter_args` handling in tile (and fuse).

The current approach for handling `iter_args` was to replace all uses
of the value that is used as `init` value with the corresponding
region block argument within the `scf.for`. This is not always
correct. Instead a more deliberate approach needs to be taken to
handle these. If the slice being fused represents a slice of the
destination operand of the untiled op, then
- Make the destination of the fused producer the `init` value of the
  loop nest
- For the tiled and fused producer op created, replace the slice of
  the destination operand with a slice of the corresponding region
  iter arg of the innermost loop of the generated loop nest

Differential Revision: https://reviews.llvm.org/D134411