fix a crash/hang in SimplifyCFG.

Jump threading in an unreachable CFG region can lead to a crash (in an assert compiler) or hang (in an no-assert compiler) in `ValueBase::replaceAllUsesWith`.

Unfortunately I couldn't come up with an isolated SIL test case.

rdar://92267349

Author: eeckstein

lib/SILOptimizer/Transforms/SimplifyCFG.cpp

@@ -1319,8 +1319,14 @@ TrampolineDest::TrampolineDest(SILBasicBlock *sourceBB,
       return;
     }
   }
+  SILBasicBlock *destBlock = targetBranch->getDestBB();
   newSourceBranchArgs.reserve(targetBranch->getArgs().size());
   for (SILValue branchArg : targetBranch->getArgs()) {
+    if (branchArg->getParentBlock() == destBlock) {
+      // This can happen if the involved blocks are part of an unreachable CFG
+      // cycle (dominance is not meaningful in such a case).
+      return;
+    }
     if (branchArg->getParentBlock() == targetBB) {
       auto *phi = dyn_cast<SILPhiArgument>(branchArg);
       if (!phi || !phi->isPhi()) {
@@ -1331,7 +1337,7 @@ TrampolineDest::TrampolineDest(SILBasicBlock *sourceBB,
     newSourceBranchArgs.push_back(branchArg);
   }
   // Setting destBB constructs a valid TrampolineDest.
-  destBB = targetBranch->getDestBB();
+  destBB = destBlock;
 }
 
 #ifndef NDEBUG

test/SILOptimizer/simplify_cfg_crash.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -O %s -emit-sil -o /dev/null
+
+public class X {}
+
+public func testit(_ s: [X]) -> X? {
+  var r: X?
+  for e in s {
+    if r == nil {
+      r = e
+    } else if e !== r {
+      return nil
+    }
+  }
+  return r
+}
+