[InstCombine] Propagate nsw flag when negating

When pushing a sub nsw 0, %x negation into an expression, try to
preserve the nsw flag for the cases where this is possible. Do this
by passing the flag through recursive Negator::negate() calls.

Proofs: https://alive2.llvm.org/ce/z/oRPNcY

Differential Revision: https://reviews.llvm.org/D158510