SimplifyCFG: fix an infinite jump-threading loop.

The JumpThreadingCost map in Simplify CFG is used to prevent infinite jump threading loops.
There was a missing update of the cost for blocks which are cloned:
Jump threading loops were prevented for infinitely cloning the original block, but not for re-cloning the cloned block.

A test case is already added in 8948f75

rdar://73357726, [SR-14068]

Author: eeckstein

lib/SILOptimizer/Transforms/SimplifyCFG.cpp

@@ -302,6 +302,8 @@ bool SimplifyCFG::threadEdge(const ThreadInfo &ti) {
     return false;
 
   Cloner.cloneBranchTarget(SrcTerm);
+  JumpThreadingCost[Cloner.getNewBB()] = JumpThreadingCost[SrcTerm->getDestBB()];
+
 
   // We have copied the threaded block into the edge.
   auto *clonedSrc = Cloner.getNewBB();
@@ -1112,6 +1114,10 @@ bool SimplifyCFG::tryJumpThreading(BranchInst *BI) {
   Cloner.cloneBranchTarget(BI);
   Cloner.updateSSAAfterCloning();
 
+  // Also account the costs to the cloned DestBB, so the jump threading cannot
+  // loop by cloning the cloned block again.
+  JumpThreadingCost[Cloner.getNewBB()] += copyCosts;
+
   // Once all the instructions are copied, we can nuke BI itself.  We also add
   // the threaded and edge block to the worklist now that they (likely) can be
   // simplified.
@@ -2852,6 +2858,8 @@ bool SimplifyCFG::tailDuplicateObjCMethodCallSuccessorBlocks() {
     Cloner.cloneBranchTarget(Branch);
     Cloner.updateSSAAfterCloning();
 
+    JumpThreadingCost[Cloner.getNewBB()] = JumpThreadingCost[DestBB];
+
     Changed = true;
     // Simplify the cloned block and continue tail duplicating through its new
     // successors edges.