Fix a bug in getSCEVAtScope w.r.t. non-canonical loops

Fix a bug in getSCEVAtScope w.r.t. non-canonical loops

The issue is that if we have a loop with multiple predecessors outside the loop, the code was expecting to merge them and only return if equal, but instead returned the first one seen.

I have no idea if this actually tripped anywhere.  I noticed it by accident when reading the code and have no idea how to go about constructing a test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@363112 91177308-0d34-0410-b5e6-96231b3b80d8