[SafepointIRVerifier] NFC: Refactor code for identifying exclusive base type

Added a new Enum to identify if the base pointer is exclusively null or
exlusively some constant or not exclusively any constant.
Converted the base pointer identification method from recursive to
iterative form.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307340 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed