GraphWriter: tweak the program fallback order

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210003 91177308-0d34-0410-b5e6-96231b3b80d8
Silence -Wreturn-type warning

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210005 91177308-0d34-0410-b5e6-96231b3b80d8