[IPSCCP] Support unfeasible default dests for switch.

At the moment, unfeasible default destinations are not handled properly
in removeNonFeasibleEdges. So far, only unfeasible cases are removed,
but later code expects unreachable blocks to have no predecessors.

This is causing the crash reported in PR49573.

If the default destination is unfeasible it won't be executed. Create
a new unreachable block on demand and use that as default
destination.

Note that at the moment this only is relevant for cases where
resolvedUndefsIn marks the first case as executable. Regular switch
handling has a FIXME/TODO to support determining whether the default
case is feasible or not.

Fixes #48917.

Differential Revision: https://reviews.llvm.org/D113497