[mlir][tensor][bufferize] Support memory_space for tensor.pad

This change adds memory space support to tensor.pad. (tensor.generate and tensor.from_elements do not support memory spaces yet.)

The memory space is inferred from the buffer of the source tensor.

Instead of lowering tensor.pad to tensor.generate + tensor.insert_slice, it is now lowered to bufferization.alloc_tensor (with the correct memory space) + linalg.map + tensor.insert_slice.

Memory space support for the remaining two tensor ops is left for a later point, as this requires some more design discussions.

Differential Revision: https://reviews.llvm.org/D136265