diff --git a/llvm/lib/Transforms/Utils/FlattenCFG.cpp b/llvm/lib/Transforms/Utils/FlattenCFG.cpp
index ddd3f597ae0191..89823ed1c831ac 100644
--- a/llvm/lib/Transforms/Utils/FlattenCFG.cpp
+++ b/llvm/lib/Transforms/Utils/FlattenCFG.cpp
@@ -431,6 +431,9 @@ bool FlattenCFGOpt::MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder) {
     return false;
 
   BasicBlock *FirstEntryBlock = CInst1->getParent();
+  // Don't die trying to process degenerate/unreachable code.
+  if (FirstEntryBlock == SecondEntryBlock)
+    return false;
 
   // Either then-path or else-path should be empty.
   bool InvertCond2 = false;
diff --git a/llvm/test/Transforms/Util/flattencfg.ll b/llvm/test/Transforms/Util/flattencfg.ll
index e94e0cdc939df0..3e598ee7c9cbc1 100644
--- a/llvm/test/Transforms/Util/flattencfg.ll
+++ b/llvm/test/Transforms/Util/flattencfg.ll
@@ -216,3 +216,20 @@ if.else.y:
 exit:
   ret void
 }
+
+; This would crash.
+
+declare i1 @llvm.smax.i1(i1, i1) #0
+
+; CHECK-LABEL: @PR56875
+define void @PR56875(i1 %val_i1_5) {
+entry_1:
+  ret void
+
+bb_2:                                             ; preds = %bb_4
+  br label %bb_4
+
+bb_4:                                             ; preds = %bb_4, %bb_2
+  %val_i1_46 = call i1 @llvm.smax.i1(i1 %val_i1_5, i1 %val_i1_5)
+  br i1 %val_i1_46, label %bb_4, label %bb_2
+}