[clang] [clang][dataflow] Check for backedges directly (instead of loop statements). (PR #68923)

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun approved this pull request.

Nice fix! So, it looks like our definition of "loop head" was wrong. 

https://github.com/llvm/llvm-project/pull/68923