LoopRotate: Avoid introducing new critical edges during rotation.

Author: atrick

lib/SILOptimizer/LoopTransforms/LoopRotate.cpp

@@ -409,10 +409,13 @@ bool swift::rotateLoop(SILLoop *loop, DominanceInfo *domInfo,
   header->moveAfter(latch);
 
   // Merge the old latch with the old header if possible.
-  mergeBasicBlockWithSuccessor(latch, domInfo, loopInfo);
+  if (mergeBasicBlockWithSuccessor(latch, domInfo, loopInfo))
+    newHeader = latch; // The old Header is gone. Latch is now the Header.
 
-  // Create a new preheader.
-  splitIfCriticalEdge(preheader, newHeader, domInfo, loopInfo);
+  // Cloning the header into the preheader created critical edges from the
+  // preheader and original header to both the new header and loop exit.
+  splitCriticalEdgesFrom(preheader, domInfo, loopInfo);
+  splitCriticalEdgesFrom(newHeader, domInfo, loopInfo);
 
   if (shouldVerify) {
     domInfo->verify();