Rollup merge of #69158 - ecstatic-morse:graphviz-diff, r=matthewjasper

Rollup merge of #69158 - ecstatic-morse:graphviz-diff, r=matthewjasper

Don't print block exit state in dataflow graphviz if unchanged

A small quality-of-life improvement I was using while working on #68528. It's pretty common to have a lot of zero-statement basic blocks, especially before a `SimplifyCfg` pass is run. When the dataflow state was dense, these blocks could take up a lot of vertical space since the full flow state was printed on both entry and exit. After this PR, we only print a block's exit state if it differs from that block's entry state. Take a look at the two basic blocks on the left.

Before:

![image](https://user-images.githubusercontent.com/29463364/74505395-e2d1dd00-4eab-11ea-8006-ec8f0dc9d1b6.png)

After:
![image](https://user-images.githubusercontent.com/29463364/74505277-98506080-4eab-11ea-8d95-5190bc378331.png)