AST. Collapse subclasses of IncrementOrDecrementExpression into it, with 2 enums.

It turned out that the split was not useful, and we had to invoke a
very similar code in each place. Well, I was warned about it.

Change-Id: I7bb1133731f253487c1c63ea9995a9d063d736d1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/544920
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>