[SimplifyCFG] FoldBranchToCommonDest(): re-lift restrictions on liveout uses of bonus...

[SimplifyCFG] FoldBranchToCommonDest(): re-lift restrictions on liveout uses of bonus instructions

I have previously tried doing that in
b33fbbaa34f0fe9fb16789afc72ae424c1825b69 / d38205144febf4dc42c9270c6aa3d978f1ef65e1,
but eventually it was pointed out that the approach taken there
was just broken wrt how the uses of bonus instructions are updated
to account for the fact that they should now use either bonus instruction
or the cloned bonus instruction. In particluar, all that manual handling
of PHI nodes in successors was just wrong.

But, the fix is actually much much simpler than my initial approach:
just tell SSAUpdate about both instances of bonus instruction,
and let it deal with all the PHI handling.

Alive2 confirms that the reproducers from the original bugs (@pr48450*)
are now handled correctly.

This effectively reverts commit 59560e85897afc50090b6c3d920bacfd28b49d06,
effectively relanding b33fbbaa34f0fe9fb16789afc72ae424c1825b69.