[mlir][sparse] generalize sparse_tensor.convert on static/dynamic dimension sizes

This revison lifts the artificial restriction on having exact matches between
source and destination type shapes. A static size may become dynamic. We still
reject changing a dynamic size into a static size to avoid the need for a
runtime "assert" on the conversion. This revision also refactors some of the
conversion code to share same-content buffers.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D111915