Simplify the "impossible branch" handling, and fix a bug in the
process.  CmmSwitch encodes the possibility of having impossible
branches (the destinations are Maybe BlockId rather than just BlockId)
so we don't need to encode impossible branches as dummy blocks
containing a jump to an impossible location (currently 0).

However, PprC and PprCmm weren't set up to cope with Nothings in a
CmmSwitch, so this commit fixes that too.