[mlir][Vector] Degrade masking information when forwarding linalg.copy to vector.transfer

Summary:
linalg.copy + linalg.fill can be used to create a padded local buffer.
The `masked` attribute is only valid on this padded buffer.
When forwarding to vector.transfer ops, the attribute must be reset
conservatively.

Differential Revision: https://reviews.llvm.org/D83782