[SPIRV] Fix assertion violation caused by unexpected ConstantExpr. (#170524)

`SPIRVEmitIntrinsics::simplifyZeroLengthArrayGepInst` asserted that it
always expected a `GetElementPtrInst` from `IRBuilder::CreateGEP` (which
returns a `Value`). `IRBuilder` can fold and return a `ConstantExpr`
instead, thus violating the assertion. The patch fixes this by using
`GetElementPtrInst::Create` to always return a `GetElementPtrInst`.

This LLVM defect was identified via the AMD Fuzzing project.

Author: mgcarrasco

llvm/lib/Target/SPIRV/SPIRVEmitIntrinsics.cpp

@@ -2834,13 +2834,11 @@ SPIRVEmitIntrinsics::simplifyZeroLengthArrayGepInst(GetElementPtrInst *GEP) {
   ArrayType *ArrTy = dyn_cast<ArrayType>(SrcTy);
   if (ArrTy && ArrTy->getNumElements() == 0 &&
       PatternMatch::match(Indices[0], PatternMatch::m_Zero())) {
-    IRBuilder<> Builder(GEP);
     Indices.erase(Indices.begin());
     SrcTy = ArrTy->getElementType();
-    Value *NewGEP = Builder.CreateGEP(SrcTy, GEP->getPointerOperand(), Indices,
-                                      "", GEP->getNoWrapFlags());
-    assert(llvm::isa<GetElementPtrInst>(NewGEP) && "NewGEP should be a GEP");
-    return cast<GetElementPtrInst>(NewGEP);
+    return GetElementPtrInst::Create(SrcTy, GEP->getPointerOperand(), Indices,
+                                     GEP->getNoWrapFlags(), "",
+                                     GEP->getIterator());
   }
   return nullptr;
 }

llvm/test/CodeGen/SPIRV/const-array-gep.ll

@@ -0,0 +1,19 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: llc -O0 -mtriple=spirv64-unknown-unknown < %s | FileCheck %s
+; RUN: %if spirv-tools %{ llc -O0 -mtriple=spirv64-unknown-unknown < %s -filetype=obj | spirv-val %}
+
+define spir_kernel void @_Z6kernelPi() addrspace(4) {
+; CHECK-LABEL: _Z6kernelPi
+; CHECK:       %12 = OpFunction %3 None %4 ; -- Begin function _Z6kernelPi
+; CHECK-NEXT:    %2 = OpLabel
+; CHECK-NEXT:    %13 = OpBitcast %6 %11
+; CHECK-NEXT:    %14 = OpInBoundsPtrAccessChain %6 %13 %10
+; CHECK-NEXT:    %15 = OpConvertPtrToU %5 %14
+; CHECK-NEXT:    %16 = OpBitcast %6 %11
+; CHECK-NEXT:    OpStore %16 %15 Aligned 4
+; CHECK-NEXT:    OpReturn
+; CHECK-NEXT:    OpFunctionEnd
+entry:
+  store i32 ptrtoint (ptr addrspace(4) getelementptr inbounds ([0 x i32], ptr addrspace(4) null, i64 0, i64 1) to i32), ptr addrspace(4) null, align 4
+  ret void
+}