[InstCombine] use 'match' to reduce code; NFCI

The later uses of dyn_castNotVal in this block are either
incomplete (doesn't handle vector constants) or overstepping
(shouldn't handle constants at all), but this first use is
just unnecessary. 'I' is obviously not a constant, and it 
can't be a not-of-a-not because that would already be
instsimplified.

llvm-svn: 301088