[mlir][sparse][bufferization] refine bufferization assumption enforcement

Enforce the assumption made on tensor buffers explicitly. When in-place,
reuse the buffer, but fill with all zeroes for the non-update case, since
the kernel assumes all elements are written to. When not in-place, zero
out the new buffer when materializing or when no-updates occur. Copy the
original tensor value when updates occur. This prepares migrating to the
new bufferization strategy, where these assumptions must be made explicit.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D128691