Complete TODOs / cleanup for loop-fusion utility

- this is CL 1/2 that does a clean up and gets rid of one limitation in an
  underlying method - as a result, fusion works for more cases.
- fix bugs/incomplete impl. in toAffineMapFromEq
- fusing across rank changing reshapes for example now just works

  For eg. given a rank 1 memref to rank 2 memref reshape (64 -> 8 x 8) like this,
  -loop-fusion -memref-dataflow-opt now completely fuses and inlines/store-forward
  to get rid of the temporary:

INPUT

  // Rank 1 -> Rank 2 reshape
  for %i0 = 0 to 64 {
     %v = load %A[%i0]
     store %v, %B[%i0 floordiv 8, i0 mod 8]
  }

  for %i1 = 0 to 8
    for %i2 = 0 to 8
      %w = load %B[%i1, i2]
      "foo"(%w) : (f32) -> ()

OUTPUT

$ mlir-opt -loop-fusion -memref-dataflow-opt fuse_reshape.mlir

#map0 = (d0, d1) -> (d0 * 8 + d1)
mlfunc @fuse_reshape(%arg0: memref<64xf32>) {
  for %i0 = 0 to 8 {
    for %i1 = 0 to 8 {
      %0 = affine_apply #map0(%i0, %i1)
      %1 = load %arg0[%0] : memref<64xf32>
      "foo"(%1) : (f32) -> ()
    }
  }
}

AFAIK, there is no polyhedral tool / compiler that can perform such fusion -
because it's not really standard loop fusion, but possible through a
generalized slicing-based approach such as ours.

PiperOrigin-RevId: 227918338