[CIR][LowerToLLVM] Fix crash in PtrStrideOp lowering

Assumptions about values having a defining op can be misleading when block
arguments are involved.

Author: bcardosolopes

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

@@ -499,21 +499,21 @@ class CIRPtrStrideOpLowering
     // Zero-extend, sign-extend or trunc the pointer value.
     auto index = adaptor.getStride();
     auto width = index.getType().cast<mlir::IntegerType>().getWidth();
-    mlir::DataLayout LLVMLayout(
-        index.getDefiningOp()->getParentOfType<mlir::ModuleOp>());
+    mlir::DataLayout LLVMLayout(ptrStrideOp->getParentOfType<mlir::ModuleOp>());
     auto layoutWidth =
         LLVMLayout.getTypeIndexBitwidth(adaptor.getBase().getType());
-    if (layoutWidth && width != *layoutWidth) {
+    auto indexOp = index.getDefiningOp();
+    if (indexOp && layoutWidth && width != *layoutWidth) {
       // If the index comes from a subtraction, make sure the extension happens
       // before it. To achieve that, look at unary minus, which already got
       // lowered to "sub 0, x".
-      auto sub = dyn_cast<mlir::LLVM::SubOp>(index.getDefiningOp());
+      auto sub = dyn_cast<mlir::LLVM::SubOp>(indexOp);
       auto unary =
           dyn_cast<mlir::cir::UnaryOp>(ptrStrideOp.getStride().getDefiningOp());
       bool rewriteSub =
           unary && unary.getKind() == mlir::cir::UnaryOpKind::Minus && sub;
       if (rewriteSub)
-        index = index.getDefiningOp()->getOperand(1);
+        index = indexOp->getOperand(1);
 
       // Handle the cast
       auto llvmDstType = mlir::IntegerType::get(ctx, *layoutWidth);

clang/test/CIR/Lowering/ptrstride.cir

@@ -12,6 +12,10 @@ module {
     %4 = cir.load %3 : !cir.ptr<!s32i>, !s32i
     cir.return
   }
+  cir.func @g(%arg0: !cir.ptr<!s32i>, %2 : !s32i) {
+    %3 = cir.ptr_stride(%arg0 : !cir.ptr<!s32i>, %2 : !s32i), !cir.ptr<!s32i>
+    cir.return
+  }
 }
 
 // MLIR-LABEL: @f
@@ -24,3 +28,6 @@ module {
 // MLIR:   %[[VAL_6:.*]] = llvm.getelementptr %[[VAL_3]]{{\[}}%[[VAL_5]]] : (!llvm.ptr, i64) -> !llvm.ptr, i32
 // MLIR:   %[[VAL_7:.*]] = llvm.load %[[VAL_6]] {alignment = 4 : i64} : !llvm.ptr -> i32
 // MLIR:   llvm.return
+
+// MLIR-LABEL: @g
+// MLIR: llvm.getelementptr %arg0[%arg1] : (!llvm.ptr, i32) -> !llvm.ptr, i32