Update replaceAllMemRefUsesWith to generate single result affine_apply's for

index remapping
- generate a sequence of single result affine_apply's for the index remapping
  (instead of one multi result affine_apply)
- update dma-generate and loop-fusion test cases; while on this, change test cases
  to use single result affine apply ops
- some fusion comment fix/cleanup

PiperOrigin-RevId: 230985830