[mlir][sparse] enhance sparse reduction support

Formerly, we accepted and/prod reductions as a standard
reduction but these change the semantics after sparsification
by not looking at implicit zeros. Therefore, we only accept
standard reductions that are insensitive to implicit vs.
explicit zeros, and leave the more complex reductions to
the sparse_tensor.reduce custom reduction implementation.

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D151929