[SCEVExp] Keep NUW/NSW if both original inc and isomporphic inc agree. (#79512)

We are replacing with a wider increment. If both OrigInc and
IsomorphicInc are NUW/NSW, then we can preserve them on the wider
increment; the narrower IsomorphicInc would wrap before the wider
OrigInc, so the replacement won't make IsomorphicInc's uses more
poisonous.

PR: https://github.com/llvm/llvm-project/pull/79512