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

[Yitzhak Mandelbaum via cfe-commits]

ymand wrote:

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

Indeed. We didn't think of the way control-flow constructs *inside* the condition expression would be represented. Ironically (?), this is actually simpler and fixes another issue besides, which is nice. ;)

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