Allow 4 as a valid debug info version.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187763 91177308-0d34-0410-b5e6-96231b3b80d8
Factor FlattenCFG out from SimplifyCFG

Patch by: Mei Ye

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187764 91177308-0d34-0410-b5e6-96231b3b80d8