[mlir] Make patterns for folding tensor.empty optional.

At the moment, they are a part of EmptyOp::getCanonicalizationPatterns. When
extract_slice(tensor.empty) is rewritten as a new tensor.empty, it could
happen that we end up with two tensor.empty ops, since the original
tensor.empty can have two users. After bufferization such cases result in two
allocations.

Differential Revision: https://reviews.llvm.org/D139308