Only print edgeSourceLabels if they are not empty

Only print edgeSourceLabels if they are not empty

Graphviz can layout the graphs better if a node does not contain source
ports. Therefore only print the ports if the source ports are useful,
that means are not labeled with the empty string "".
This patch also simplifies graphs without any edgeSourceLabels e.g. the
dominance trees.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@90131 91177308-0d34-0410-b5e6-96231b3b80d8