SCCIterator: Merge MinVisitNumStack and VisitStack

This patch merges MinVisitNumStack with VisitStack using a StackElement
struct.

Patch by Mehdi Amini!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201353 91177308-0d34-0410-b5e6-96231b3b80d8