[clang] [CIR] Fix remaining (part 2) FlattenCFG rewriter contract violations (PR #192503)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

@andykaylor definitely needs to review this one instead of me. I don't see anything other than a suggestion to extract the 'walk' operations. 

https://github.com/llvm/llvm-project/pull/192503