[XLA] Fix HloModule clone.

Currently the cloning of an instruction is usually "shallow": the called
computations of the instruction are reused in the clone. This mechanism is
useful when the hlo graph need to be modified in place (e.g. inliner, and some
hlo passes). One exception is the fusion instruction: it's always "deep"
copied, which means the fused computation is copied as well. So when we deep
cloning an HLO module, don't re-copy the fused computation, and do let the
instruction's clone function know where to put the copied fused computation.

PiperOrigin-RevId: 183181206