DebugInfo: While loop backedge should be attribute to the start of the while statement.

A little test case simplification - this could be simplified further,
though there are certainly interesting connections to the if/else
construct so I'm hesitant to remove that entirely though it does appear
somewhat unrelated.

(similar fix to r215766, related to PR19864)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215768 91177308-0d34-0410-b5e6-96231b3b80d8