diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 4a80558..c15f649 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1162,7 +1162,11 @@
     return Constant::getNullValue(Op0->getType());
 
   // X shift by 0 -> X
-  if (match(Op1, m_Zero()))
+  // Shift-by-sign-extended bool must be shift-by-0 because shift-by-all-ones
+  // would be poison.
+  Value *X;
+  if (match(Op1, m_Zero()) ||
+      (match(Op1, m_SExt(m_Value(X))) && X->getType()->isIntOrIntVectorTy(1)))
     return Op0;
 
   // Fold undefined shifts.