Fix coro lowering of single predecessor phis

Code assumes that uses of single predecessor phis are not live accross
suspend points. Cleanup any single predecessor phis preceeding the code
making this assumption.

rdar://76020301

Author: aschwaighofer

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

@@ -1480,6 +1480,24 @@ static void rewritePHIsForCleanupPad(BasicBlock *CleanupPadBB,
   }
 }
 
+static void cleanupSinglePredPHIs(Function &F) {
+  SmallVector<PHINode *, 32> Worklist;
+  for (auto &BB : F) {
+    for (auto &Phi : BB.phis()) {
+      if (Phi.getNumIncomingValues() == 1) {
+        Worklist.push_back(&Phi);
+      } else
+        break;
+    }
+  }
+  while (!Worklist.empty()) {
+    auto *Phi = Worklist.back();
+    Worklist.pop_back();
+    auto copy = Phi->getIncomingValue(0);
+    Phi->replaceAllUsesWith(copy);
+  }
+}
+
 static void rewritePHIs(BasicBlock &BB) {
   // For every incoming edge we will create a block holding all
   // incoming values in a single PHI nodes.
@@ -2269,6 +2287,10 @@ void coro::buildCoroutineFrame(Function &F, Shape &Shape) {
     }
   }
 
+  // Later code makes structural assumptions about single predecessors phis e.g
+  // that they are not live accross a suspend point.
+  cleanupSinglePredPHIs(F);
+
   // Transforms multi-edge PHI Nodes, so that any value feeding into a PHI will
   // never has its definition separated from the PHI by the suspend point.
   rewritePHIs(F);