[InstCombine] Clear out nsw/nuw if we modify computation in the chain

An nsw/nuw operation relies on the values feeding into it to not
overflow if 'poison' is not to be produced.  This means that
optimizations which make modifications to the bottom of a chain (like
SimplifyDemandedBits) must strip out nsw/nuw if they cannot ensure that
they will be preserved.

This fixes PR23309.

llvm-svn: 235544