[XLA] Replace HloComputation::ReplaceUsesOfInstruction with HloInstruction::ReplaceAllUsesWith.

RAUW used to be *almost* synonymous with RUOI, except RAUW didn't update
the computation's root.  This was a dangerous footgun -- if you
accidentally called RAUW when you wanted RUOI (which you almost always
did), your code would work perfectly, except when the relevant node
happened to be the root of a computation.

This change simplifies our APIs so there's just one Right Way To Do It,
by making RAUW update the computation.

PiperOrigin-RevId: 170290230