[XLA] Minor cleanups related to multi-output fusion.

- Add some comments about preexisting invariants, and add some CHECKs.

- In the LoopEmitter constructor, materialize the given
  ArraySlice<IrArray> to a vector, so we don't rely on the given
  ArraySlice having any particular lifetime.

- Add the invariant that the LoopEmitter constructor which takes a
  list of IrArrays is only for multi-output fusion.  Previously it said:
  If you only pass one array, then treat it as regular fusion.  But this
  results in an LLVM type mismatch, because the given
  target_element_generator should be passing a struct with one element.

PiperOrigin-RevId: 184365310