[mlir][sparse] fix asan issue

The LinalgElementwiseOpFusion pass has become smarter, and converts
the simple conversion linalg operation into a sparse dialect convert
operation. However, since our current bufferization does not take the
new semantics into consideration, we leak memory of the allocation.
For now, this has been fixed by making the operation less trivial.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D128002