diff --git a/llvm/lib/Transforms/Utils/FixIrreducible.cpp b/llvm/lib/Transforms/Utils/FixIrreducible.cpp
--- a/llvm/lib/Transforms/Utils/FixIrreducible.cpp
+++ b/llvm/lib/Transforms/Utils/FixIrreducible.cpp
@@ -138,10 +138,18 @@
     // not be necessary if we can retain such backedges.
     if (Headers.count(Child->getHeader())) {
       for (auto BB : Child->blocks()) {
+        if (LI.getLoopFor(BB) != Child)
+          continue;
         LI.changeLoopFor(BB, NewLoop);
         LLVM_DEBUG(dbgs() << "moved block from child: " << BB->getName()
                           << "\n");
       }
+      std::vector<Loop *> GrandChildLoops;
+      std::swap(GrandChildLoops, Child->getSubLoopsVector());
+      for (auto GrandChildLoop : GrandChildLoops) {
+        GrandChildLoop->setParentLoop(nullptr);
+        NewLoop->addChildLoop(GrandChildLoop);
+      }
       LI.destroy(Child);
       LLVM_DEBUG(dbgs() << "subsumed child loop (common header)\n");
       continue;
diff --git a/llvm/test/Transforms/FixIrreducible/issue55099.ll b/llvm/test/Transforms/FixIrreducible/issue55099.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/FixIrreducible/issue55099.ll
@@ -0,0 +1,39 @@
+; RUN: opt %s -fix-irreducible -S -o - | FileCheck %s
+
+define dso_local void @foo(i1 %a) {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label %irr.guard
+; CHECK:       block1:
+; CHECK-NEXT:    br label %irr.guard
+; CHECK:       block2:
+; CHECK-NEXT:    br label %body
+; CHECK:       body:
+; CHECK-NEXT:    br i1 %a, label %latch, label %inner_body
+; CHECK:       inner_body:
+; CHECK-NEXT:    br label %body
+; CHECK:       latch:
+; CHECK-NEXT:    br label %irr.guard
+; CHECK:       irr.guard:
+; CHECK-NEXT:    [[GUARD:%.*]] = phi i1 [ %a, %latch ], [ %a, %block1 ], [ %a, %entry ]
+; CHECK-NEXT:    br i1 [[GUARD]], label %block1, label %block2
+;
+entry:
+  br i1 %a, label %block1, label %block2
+
+block1:
+  br i1 %a, label %block1, label %block2
+
+block2:
+  br label %body
+
+body:
+  br i1 %a, label %latch, label %inner_body
+
+inner_body:
+  br label %body
+
+latch:
+  br i1 %a, label %block1, label %block2
+
+}