[DAG][AArch64] Limit preferIncOfAddToSubOfNot until after legalization if the node has wrap flags

If the add node has wrap flags then they will be destroyed by converting to
sub/not. The flags can be useful in converting to rhadd, for example, but that
may be required late if the node types need to be legalized. This limits the
preferIncOfAddToSubOfNot fold until after legalize DAG if the node have flags
to allow more folding.

Differential Revision: https://reviews.llvm.org/D148809