DOTGraphTraits: Allow the decision to show a graph to consider the analysis

The method processFunction() is called to decide if a graph should be shown for
a certain function. To allow DOTGraphTraitViewers to take this decision based
on the analysis results for the given function, we forward a reference to the
analysis result. This will be used by Polly to only visualize functions where
interesting loop regions have been detected.

llvm-svn: 255889