[SLP] Visualize SLP trees with -view-slp-tree

Analyzing larger trees is extremely difficult with the current debug output so
this adds GraphTraits and DOTGraphTraits on top of the VectorizableTree data
structure.  We can now display the SLP trees with Graphviz as in
https://reviews.llvm.org/F3132765.

I decorated the graph where a value needs to be gathered for one reason or
another.  These are the red nodes.

There are other improvement I am planning to make as I work through my case
here.  For example, I would also like to mark nodes that need to be extracted.

Differential Revision: https://reviews.llvm.org/D30731

llvm-svn: 297303