[InstCombine] remove dead cases from icmp pair switches; NFCI

"PredicatesFoldable" returns false for signed/unsigned mismatched pairs,
so these cases should never exist. We'll default to 'unreachable' on those 
predicate combos instead.

Most of what's left in these switches belongs in InstSimplify (and may 
already be there), so there's probably more that can be done to reduce
this code.

llvm-svn: 299829