i965: Refactor Sandybridge implied move handling.

i965: Refactor Sandybridge implied move handling.

This was open-coded in three different places, and more are necessary.
Extract this into a function so it can be reused.

Unfortunately, not all variations were the same: in particular, one set
compression control and checked that the source register was not
ARF_NULL.  This seemed like a good idea, so all cases now do so.