[CSSPGO][SCCIterator] Fix a non-determinism in scc_member_iterator

Previously we initialed the work queue with MST roots based on NodeInfoMap which is an unordered map. This could cause a non-determinism. I'm fixing this by initializing the queue based on SortedEdges.

I don't see any performance move with this change. However this helps debugging.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D120670