GraphWriter: tweak the program fallback order

Amend r210001 to use the classic fallback order behaviour if the requested
graphing program isn't found.

llvm-svn: 210003
diff --git a/llvm/lib/Support/GraphWriter.cpp b/llvm/lib/Support/GraphWriter.cpp
index b707a2d..c26f6a1 100644
--- a/llvm/lib/Support/GraphWriter.cpp
+++ b/llvm/lib/Support/GraphWriter.cpp
@@ -176,7 +176,7 @@
   std::string GeneratorPath;
   if (PSViewer &&
       (S.TryFindProgram(getProgramName(program), GeneratorPath) ||
-       S.TryFindProgram("circo|twopi|neato|fdp|dot", GeneratorPath))) {
+       S.TryFindProgram("dot|fdp|neato|twopi|circo", GeneratorPath))) {
     std::string PSFilename = Filename + ".ps";
 
     std::vector<const char *> args;