[mlir][scf] Simplify the logic for `replaceLoopWithNewYields` for perfectly nested loops.

Based on discussion in https://reviews.llvm.org/D134411, instead of
first modifying the inner most loop first followed by modifying the
outer loops from inside out, this patch restructures the logic to
start the modification from the outer most loop.

Differential Revision: https://reviews.llvm.org/D134832