[SimplifyCFG] SwitchToLookupTable(): don't insert a DomTree edge that already exists

SI is the terminator of BB, so the edge we are adding obviously already existed.

Indeed, this change does not have a test coverage change.
This failure has been exposed in an existing test coverage
by a follow-up patch that switches to lazy domtreeupdater mode,
and removes domtree verification from
SimplifyCFGOpt::simplifyOnce()/SimplifyCFGOpt::run(),
IOW it does not appear feasible to add dedicated test coverage here.