[InstCombine] remove an icmp fold that is already handled by InstSimplify

Specifically, this is done near the end of "SimplifyICmpInst" using 
computeKnownBits() as the broader solution. There are even vector
tests (yay!) for this in test/Transforms/InstSimplify/compare.ll.

I considered putting an assert here instead of just deleting, but
then we could assert every possible fold in InstSimplify in 
InstCombine, so...less is more?

llvm-svn: 279300