Index: llvm/lib/Transforms/Scalar/LoopInstSimplify.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopInstSimplify.cpp
+++ llvm/lib/Transforms/Scalar/LoopInstSimplify.cpp
@@ -104,6 +104,10 @@
           auto *UserI = cast<Instruction>(U.getUser());
           U.set(V);
 
+          // Do not bother dealing with unreachable code.
+          if (!DT.isReachableFromEntry(UserI->getParent()))
+            continue;
+
           // If the instruction is used by a PHI node we have already processed
           // we'll need to iterate on the loop body to converge, so add it to
           // the next set.
Index: llvm/test/Transforms/LoopInstSimplify/pr55072.ll
===================================================================
--- llvm/test/Transforms/LoopInstSimplify/pr55072.ll
+++ llvm/test/Transforms/LoopInstSimplify/pr55072.ll
@@ -1,14 +1,35 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -S %s -passes=loop-instsimplify | FileCheck %s
 ; RUN: opt -S %s -passes='loop-mssa(loop-instsimplify)' -verify-memoryssa | FileCheck %s
 
-; XFAIL: *
-; REQUIRES: asserts
-
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128-ni:1-p2:32:8:8:32-ni:2"
 target triple = "x86_64-unknown-linux-gnu"
 
 define i32 @test_01() {
-; CHECK-LABEL: test_01
+; CHECK-LABEL: @test_01(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    br label [[BB1:%.*]]
+; CHECK:       bb1:
+; CHECK-NEXT:    br label [[BB3:%.*]]
+; CHECK:       bb2:
+; CHECK-NEXT:    br label [[BB2:%.*]]
+; CHECK:       bb3:
+; CHECK-NEXT:    switch i32 undef, label [[BB8:%.*]] [
+; CHECK-NEXT:    i32 0, label [[BB4:%.*]]
+; CHECK-NEXT:    i32 1, label [[BB6:%.*]]
+; CHECK-NEXT:    i32 2, label [[BB5:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       bb4:
+; CHECK-NEXT:    br label [[BB6]]
+; CHECK:       bb5:
+; CHECK-NEXT:    br label [[BB8]]
+; CHECK:       bb6:
+; CHECK-NEXT:    br label [[BB8]]
+; CHECK:       bb7:
+; CHECK-NEXT:    ret i32 0
+; CHECK:       bb8:
+; CHECK-NEXT:    br label [[BB3]]
+;
 bb:
   br label %bb1
 
@@ -20,9 +41,9 @@
 
 bb3:                                              ; preds = %bb8, %bb1
   switch i32 undef, label %bb8 [
-    i32 0, label %bb4
-    i32 1, label %bb6
-    i32 2, label %bb5
+  i32 0, label %bb4
+  i32 1, label %bb6
+  i32 2, label %bb5
   ]
 
 bb4:                                              ; preds = %bb3