SimplifyCFG: check uses of constant-foldable instrs in switch destinations (PR20210)

The previous code assumed that such instructions could not have any uses
outside CaseDest, with the motivation that the instruction could not
dominate CommonDest because CommonDest has phi nodes in it. That simply
isn't true; e.g., CommonDest could have an edge back to itself.

llvm-svn: 225552