[clang] [KeyInstr][Clang] For stmt atom (PR #134646)

[Orlando Cazalet-Hyams via cfe-commits]

OCHyams wrote:

You were right about any ctrl-flow breaking the br-to-cond atom group. Fixed, improved testing, and follow up with additional work for continue/break in https://github.com/llvm/llvm-project/pull/141618

n.b. this is now based on top of https://github.com/llvm/llvm-project/pull/141607 so please ignore the first commit (sorry, the graphite stack has completely toppled and we're back into GitHub freeform stacking nastiness)

https://github.com/llvm/llvm-project/pull/134646