[XLA] Fix invalid assumption in HloComputation::CloneWithReplacements.

CloneWithReplacements assumed that the `extras` instructions came before all
other instructions in the postorder traversal.  This is not true in general.

In the course of fixing this, I ended up removing the `extras` parameter
entirely, which makes the function easier to call.  I also augmented the
interface a bit to make CloneWithReplacements easier to call when you just have
one or two replacements to make.  I will use these overloads in a future patch.

PiperOrigin-RevId: 220390779