[mlir] Fix loop unrolling: properly replace the arguments of the epilogue loop.

Using "replaceUsesOfWith" is incorrect because the same initializer value may appear multiple times.

For example, if the epilogue is needed when this loop is unrolled
```
%x:2 = scf.for ... iter_args(%arg1 = %c1, %arg2 = %c1) {
  ...
}
```
then both epilogue's arguments will be incorrectly renamed to use the same result index (note #1 in both cases):
```
%x_unrolled:2 = scf.for ... iter_args(%arg1 = %c1, %arg2 = %c1) {
  ...
}
%x_epilogue:2 = scf.for ... iter_args(%arg1 = %x_unrolled#1, %arg2 = %x_unrolled#1) {
  ...
}
```