[mlir][sparse] Replace the folding of nop convert with a codegen rule.

[mlir][sparse] Replace the folding of nop convert with a codegen rule.

This is to allow the use of a nop convert to express that the sparse tensor
allocated through bufferization::AllocTensorOp will be expanded to sparse
tensor storage by sparse tensor codegen.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D136214