[mlir][linalg] fix indexOp folder to work in genericOp build with createOrFold (llvm#137427)

Currently in torch-mlir, indexOp folder is segfaulting when we call
createOrFold in a genericOp builder. (*this)->getParentOp ended up with
null which is causing the issue.

This is seen in poolSizeCalculator.getPoolSize
being called from createAvgPoolValueCountIncludePadFalseCase. link:

https://github.com/llvm/torch-mlir/blob/80a3dfddd341c72ab9bd6c6688b872bf3a5e4ddb/lib/Conversion/TorchToLinalg/Pooling.cpp#L918-L921

---------

Signed-off-by: Stanley Winata <stanley.winata@amd.com>
Co-authored-by: Jakub Kuderski <kubakuderski@gmail.com>

Author: raikonenfnu

mlir/lib/Dialect/Linalg/IR/LinalgOps.cpp

@@ -2284,7 +2284,12 @@ LogicalResult IndexOp::verify() {
 }
 
 OpFoldResult IndexOp::fold(FoldAdaptor adaptor) {
-  auto linalgOp = cast<LinalgOp>((*this)->getParentOp());
+  auto linalgOp = dyn_cast_or_null<LinalgOp>((*this)->getParentOp());
+  // Bail out if `linalg.index` does not have a proper parent yet at this
+  // point, e.g., when calling `createOrFold` during IR construction in
+  // `genericOp::build`.
+  if (!linalgOp)
+    return OpFoldResult{};
 
   // Index of unit dims is always 0.
   SmallVector<int64_t, 4> loopBounds = linalgOp.getStaticLoopRanges();