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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301088 91177308-0d34-0410-b5e6-96231b3b80d8