[NFCI][SimplifyCFG] Rewrite `createUnreachableSwitchDefault()`

The only thing that function should do as per it's semantic,
is to ensure that the switch's default is a block consisting only of
an `unreachable` terminator.

So let's just create such a block and update switch's default
to point to it. There should be no need for all this weird dance
around predecessors/successors.