[SCEV] Retain AddExpr flags when subtracting a foldable constant.

Currently we drop wrapping flags for expressions like (A + C1)<flags> - C2.

But we can retain flags under certain conditions:

* Adding a smaller constant is NUW if the original AddExpr was NUW.

* Adding a constant with the same sign and small magnitude is NSW, if the
  original AddExpr was NSW.

This can improve results after using `SimplifyICmpOperands`, which may
subtract one in order to use stricter predicates, as is the case for
`isKnownPredicate`.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D104319