[LCG] Now that the loop structure of the core SCC finding routine is

[LCG] Now that the loop structure of the core SCC finding routine is
factored into a more reasonable form, replace the tail call with
a simple outer-loop continuation. It's sad that C++ makes this so
awkward to write, but it seems more direct and clear than the tail call
at this point.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207201 91177308-0d34-0410-b5e6-96231b3b80d8