Add de-fuser pass which replaces fusion nodes with their non-fused

equivalents. This pass will be used in tools added in followup CLs.
The motivation for this pass is running an HLO module which was built
for one backend on a different backend for testing and debugging. De-fusing
is necessary because different backends have different fusion support.

Also, fix problem with deletion of fused computation identified when testing
the defuser.

PiperOrigin-RevId: 173442671