[ValueTracking] fix isOnlyUsedInZeroEqualityComparison with no users

This is another problem exposed by:
https://bugs.llvm.org/PR50836

llvm/lib/Analysis/ValueTracking.cpp

@@ -280,8 +280,7 @@ bool llvm::haveNoCommonBitsSet(const Value *LHS, const Value *RHS,
 }
 
 bool llvm::isOnlyUsedInZeroEqualityComparison(const Instruction *I) {
-  // FIXME: Should not return true if there are no users.
-  return all_of(I->users(), [](const User *U) {
+  return !I->user_empty() && all_of(I->users(), [](const User *U) {
     ICmpInst::Predicate P;
     return match(U, m_ICmp(P, m_Value(), m_Zero())) && ICmpInst::isEquality(P);
   });

llvm/test/Transforms/InstCombine/strlen-3.ll

@@ -0,0 +1,15 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+target datalayout = "p:64:64"
+
+; This would crash - PR50836
+
+define i64 @strlen(i32* %s) {
+; CHECK-LABEL: @strlen(
+; CHECK-NEXT:    [[R:%.*]] = call i64 @strlen(i32* noundef nonnull dereferenceable(1) [[S:%.*]])
+; CHECK-NEXT:    ret i64 0
+;
+  %r = call i64 @strlen(i32* %s)
+  ret i64 0
+}