[mlir] Insert tensor.cast only when needed when folding tensor.cast into extract_slice.

[bkramer]

Thanks

[pifon2a]

Update

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

LogicalResult matchAndRewrite(ExtractSliceOp sliceOp,
     auto castOp = sliceOp.getSource().getDefiningOp<tensor::CastOp>();
     if (!castOp)
       return failure();
 
     if (!canFoldIntoConsumerOp(castOp))
       return failure();
 
     /// Deduce the type of the result to use for the canonicalized operation.
+    auto sliceOpType = sliceOp.getType();
     RankedTensorType resultType =
         ExtractSliceOp::inferCanonicalRankReducedResultType(
-            sliceOp.getType().getRank(), sliceOp.getSourceType(),
+            sliceOpType.getRank(), sliceOp.getSourceType(),
             sliceOp.getMixedOffsets(), sliceOp.getMixedSizes(),
             sliceOp.getMixedStrides());
     Value newSlice = rewriter.create<ExtractSliceOp>(
         sliceOp.getLoc(), resultType, castOp.getSource(), sliceOp.getOffsets(),
         sliceOp.getSizes(), sliceOp.getStrides(), sliceOp.getStaticOffsets(),
         sliceOp.getStaticSizes(), sliceOp.getStaticStrides());
-    rewriter.replaceOpWithNewOp<tensor::CastOp>(sliceOp, sliceOp.getType(),
-                                                newSlice);
+    if (newSlice.getType() != sliceOpType)
+      rewriter.replaceOpWithNewOp<CastOp>(sliceOp, sliceOpType, newSlice);
     return success();
   }
 };
 
 /// Slice elements from `values` into `outValues`. `counts` represents the
 /// numbers of elements to stride in the original values for each dimension.
 /// The output values can be used to construct a DenseElementsAttr.
 template <typename IterTy, typename ElemTy>