[Mosaic GPU] Improve checks in utils.ptr_to_memref
We previously didn't check the strides and offset that appear in the desired
memref_type, and simply assumed that the offset is 0 and the memref is intended
to be contiguous. Now, we check that the offset is 0 (if you intend it to not be
zero, then perhaps just shift the pointer?) and use the strides inferred from the type.
I haven't found any bugs related to this, but it just feels like a footgun waiting
to fire.
PiperOrigin-RevId: 754844324
