[llvm] [SandboxVec][DAG] Fix unscheduled succs when nodes are scheduled (PR #184946)

[via llvm-commits]

https://github.com/vporpo commented:

> It seems a bit weird to me that the values are left as is when they "don't really make sense". I don't understand the larger picture well enough to judge though.

Yeah it is a bit strange, but we only have two options:
1) we stop updating them when "they do not make sense", or
2) we keep updating them but remove any assertions that check if the values are sane, like assertions for negative values.
I think the 1st option is better, because you still keep the safeguards under normal operation.

The larger picture is that a node's UnscheduledSuccs value is only used until the node gets scheduled (It is the value that tells you how many of your DAG successors are scheduled). After it becomes 0, we can go ahead and schedule the node. After the node gets scheduled it is no longer used or needed in any way, so we don't need to maintain it. If we ever need to reschedule these nodes, the UnscheduledSuccs counter will be recomputed from scratch.

https://github.com/llvm/llvm-project/pull/184946