[analyzer] Fix a bug in "collapsed" graph viewer

Nodes which have only one predecessor and only one successor can not
always be hidden, even if all states are the same.
An additional condition is needed: the predecessor may have only one successor.
This can be seen on this example:

```
  A
 / \
B   C
 \ /
  D
```

Nodes B and C can not be hidden even if all nodes in the graph have the
same state.

Differential Revision: https://reviews.llvm.org/D53735

llvm-svn: 345341