[mlir][Affine] Handle null parent op in getAffineParallelInductionVarOwner

The issue occurs during a downstream pass which does dialect conversion, where both [`FuncOpConversion`](https://github.com/llvm/llvm-project/blob/cde67b6663f994fcb4ded28fd79b23a13d347c4a/mlir/lib/Conversion/FuncToLLVM/FuncToLLVM.cpp#L480) and  [`SubviewFolder`](https://github.com/llvm/llvm-project/blob/cde67b6663f994fcb4ded28fd79b23a13d347c4a/mlir/lib/Dialect/MemRef/Transforms/ExpandStridedMetadata.cpp#L187) are run together. The original starting IR is:
```mlir
module {
  func.func @foo(%arg0: memref<100x100xf32>, %arg1: index, %arg2: index, %arg3: index, %arg4: index) -> memref<?x?xf32, strided<[100, 1], offset: ?>> {
    %subview = memref.subview %arg0[%arg1, %arg2] [%arg3, %arg4] [1, 1] : memref<100x100xf32> to memref<?x?xf32, strided<[100, 1], offset: ?>>
    return %subview : memref<?x?xf32, strided<[100, 1], offset: ?>>
  }
}
```

After `FuncOpConversion` runs, the IR looks like:
```mlir
"builtin.module"() ({
  "llvm.func"() <{CConv = #llvm.cconv<ccc>, function_type = !llvm.func<struct<(ptr, ptr, i64, array<2 x i64>, array<2 x i64>)> (ptr, ptr, i64, i64, i64, i64, i64, i64, i64, i64, i64)>, linkage = #llvm.linkage<external>, sym_name = "foo", visibility_ = 0 : i64}> ({
  ^bb0(%arg0: !llvm.ptr, %arg1: !llvm.ptr, %arg2: i64, %arg3: i64, %arg4: i64, %arg5: i64, %arg6: i64, %arg7: i64, %arg8: i64, %arg9: i64, %arg10: i64):
    %0 = "memref.subview"(<<UNKNOWN SSA VALUE>>, <<UNKNOWN SSA VALUE>>, <<UNKNOWN SSA VALUE>>, <<UNKNOWN SSA VALUE>>, <<UNKNOWN SSA VALUE>>) <{operandSegmentSizes = array<i32: 1, 2, 2, 0>, static_offsets = array<i64: -9223372036854775808, -9223372036854775808>, static_sizes = array<i64: -9223372036854775808, -9223372036854775808>, static_strides = array<i64: 1, 1>}> : (memref<100x100xf32>, index, index, index, index) -> memref<?x?xf32, strided<[100, 1], offset: ?>>
    "func.return"(%0) : (memref<?x?xf32, strided<[100, 1], offset: ?>>) -> ()
  }) : () -> ()
  "func.func"() <{function_type = (memref<100x100xf32>, index, index, index, index) -> memref<?x?xf32, strided<[100, 1], offset: ?>>, sym_name = "foo"}> ({
  }) : () -> ()
}) {llvm.data_layout = "", llvm.target_triple = ""} : () -> ()
```
The `<<UNKNOWN SSA VALUE>>`'s here are block arguments of a separate unlinked block, which is disconnected from the rest of the IR (so not only is the IR verifier-invalid, it can't even be parsed). This IR is created by signature conversion in the dialect conversion infra.

Now `SubviewFolder` is applied, and the utility function here is called on one of these disconnected block arguments, causing a crash.

It is hard to construct the reproducer in upstream because upstream does not have such setup. Adding a pass for such case is over-indexing on testing. Since it is a safer check and the method returns null to let callers handle the failure, making one-line change without tests is reasonable.

Co-authored-by: Rahul Kayaith <[email protected]>

Signed-off-by: hanhanW <[email protected]>

Author: hanhanW

mlir/lib/Dialect/Affine/IR/AffineOps.cpp

@@ -2667,7 +2667,7 @@ AffineParallelOp mlir::affine::getAffineParallelInductionVarOwner(Value val) {
   if (!ivArg || !ivArg.getOwner())
     return nullptr;
   Operation *containingOp = ivArg.getOwner()->getParentOp();
-  auto parallelOp = dyn_cast<AffineParallelOp>(containingOp);
+  auto parallelOp = dyn_cast_if_present<AffineParallelOp>(containingOp);
   if (parallelOp && llvm::is_contained(parallelOp.getIVs(), val))
     return parallelOp;
   return nullptr;