[IR] Fix accumulateConstantOffset() on zero-index GEP

These are degenerate but not malformed, so make sure we don't
crash.

Author: nikic

llvm/lib/IR/Operator.cpp

@@ -125,7 +125,7 @@ bool GEPOperator::accumulateConstantOffset(
     Type *SourceType, ArrayRef<const Value *> Index, const DataLayout &DL,
     APInt &Offset, function_ref<bool(Value &, APInt &)> ExternalAnalysis) {
   // Fast path for canonical getelementptr i8 form.
-  if (SourceType->isIntegerTy(8) && !ExternalAnalysis) {
+  if (SourceType->isIntegerTy(8) && !Index.empty() && !ExternalAnalysis) {
     auto *CI = dyn_cast<ConstantInt>(Index.front());
     if (CI && CI->getType()->isIntegerTy()) {
       Offset += CI->getValue().sextOrTrunc(Offset.getBitWidth());

llvm/unittests/IR/InstructionsTest.cpp

@@ -898,6 +898,20 @@ TEST(InstructionsTest, GEPIndices) {
   delete GEPI;
 }
 
+TEST(InstructionsTest, ZeroIndexGEP) {
+  LLVMContext Context;
+  DataLayout DL;
+  Type *PtrTy = PointerType::getUnqual(Context);
+  auto *GEP = GetElementPtrInst::Create(Type::getInt8Ty(Context),
+                                        PoisonValue::get(PtrTy), {});
+
+  APInt Offset(DL.getIndexTypeSizeInBits(PtrTy), 0);
+  EXPECT_TRUE(GEP->accumulateConstantOffset(DL, Offset));
+  EXPECT_TRUE(Offset.isZero());
+
+  delete GEP;
+}
+
 TEST(InstructionsTest, SwitchInst) {
   LLVMContext C;