Index: llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -2370,7 +2370,9 @@
     ConstantInt *ContinueReplacement =
         Direction ? ConstantInt::getFalse(BI->getContext())
                   : ConstantInt::getTrue(BI->getContext());
-    for (Value *Invariant : Invariants)
+    for (Value *Invariant : Invariants) {
+      assert(!isa<Constant>(Invariant) &&
+             "Should not be replacing constant values!");
       // Use make_early_inc_range here as set invalidates the iterator.
       for (Use &U : llvm::make_early_inc_range(Invariant->uses())) {
         Instruction *UserI = dyn_cast<Instruction>(U.getUser());
@@ -2385,6 +2387,7 @@
                  DT.dominates(ClonedPH, UserI->getParent()))
           U.set(UnswitchedReplacement);
       }
+    }
   }
 
   // We can change which blocks are exit blocks of all the cloned sibling
@@ -2727,6 +2730,9 @@
       Cond = CondNext;
     BI->setCondition(Cond);
 
+    if (isa<Constant>(Cond))
+      continue;
+
     if (L.isLoopInvariant(BI->getCondition())) {
       UnswitchCandidates.push_back({BI, {BI->getCondition()}});
       continue;
Index: llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch-invariant-select-bug.ll
===================================================================
--- llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch-invariant-select-bug.ll
+++ llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch-invariant-select-bug.ll
@@ -1,15 +1,25 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; REQUIRES: asserts
-; XFAIL: *
-; RUN: opt -passes='simple-loop-unswitch<nontrivial>' -S < %s -disable-output
+; RUN: opt -passes='simple-loop-unswitch<nontrivial>' -S < %s | FileCheck %s
 
 ; If we try to replace uses of `true` outside of `@foo`, we'll see it here.
 define i1 @bar() {
+; CHECK-LABEL: @bar(
+; CHECK-NEXT:    ret i1 true
+;
   ret i1 true
 }
 
 ; We shouldn't unswitch this loop.
 define void @foo() {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[HEADER:%.*]]
+; CHECK:       header:
+; CHECK-NEXT:    [[VAL:%.*]] = select i1 true, i1 true, i1 false
+; CHECK-NEXT:    br i1 true, label [[EXIT:%.*]], label [[HEADER]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
 entry:
   br label %header