[clang] [llvm] [clang][HLSL][SPRI-V] Add convergence intrinsics (PR #80680)

[Nathan Gauër via cfe-commits]

================
@@ -1130,8 +1130,96 @@ struct BitTest {
 
   static BitTest decodeBitTestBuiltin(unsigned BuiltinID);
 };
+
+// Returns the first convergence entry/loop/anchor instruction found in |BB|.
+// std::nullopt otherwise.
+std::optional<llvm::IntrinsicInst *> getConvergenceToken(llvm::BasicBlock *BB) {
+  for (auto &I : *BB) {
----------------
Keenuts wrote:

Yep, but I'm not sure I have any other option outside from some kind of caching(either attribute of a BB, or BB->I map). If it's this kind of alternative you are thinking about, maybe this should be left in a subsequent PR as an optimization?

https://github.com/llvm/llvm-project/pull/80680