[analyzer] Fix a bug in IdenticalExprChecker concerning while loops.

Somehow both Daniel and I missed the fact that while loops are only identical
if they have identical bodies.

Patch by Daniel Fahlgren!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201829 91177308-0d34-0410-b5e6-96231b3b80d8