reduce degree recomputation in StronglyConnectedComponents

So the computation happens every time in the do-while loop, the first time the outer while loop executes. although just once per do-while loop after, it seems, but It does reduce a lot of recomputation；because every time it jump out of the do-while loop，there are no vertices have only out-degree or in-degree，so it's no need to recompute degree to tag the vertices true.

I have done a small code proposal, because there is a problem when the pregel executions have done,  the degree no need to be recomputed.

for example，the Email-EuAll  data set：http://snap.stanford.edu/data/email-EuAll.html

do-while loop execute 10 times，and the reduce logic happend 2 times；so it would be helpful when computing StronglyConnectedComponents to reduce degree computation.

I created a branch in my fork: https://github.com/xs-li/spark/blob/master/graphx/src/main/scala/org/apache/spark/graphx/lib/StronglyConnectedComponents.scala

I hope you can consider this small code proposal.

Thank you very much,

Best regards,

xs-li