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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@90131 91177308-0d34-0410-b5e6-96231b3b80d8

dslab-epfl/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,chubbymaggie/asap,chubbymaggie/asap,chubbymaggie/asap,dslab-epfl/asap,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,chubbymaggie/asap,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,chubbymaggie/asap,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,chubbymaggie/asap,dslab-epfl/asap,GPUOpen-Drivers/llvm,dslab-epfl/asap,apple/swift-llvm,dslab-epfl/asap,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,dslab-epfl/asap