[clang] [CIR] Implement BranchOpInterface for switch.flat (PR #230340)

[via cfe-commits]

aokblast wrote:

We currently wire this as a ExternalModel as a workaround method.

See: https://github.com/Ringo-Leo/Enzyme-CIR/pull/4/changes#diff-cbce60053edef041386d48a6a5df5f8e55cbadbfe6d21411211fc2b7ee4031f6R37

https://github.com/llvm/llvm-project/pull/230340