i965/fs: Predicate byte scattered writes if needed

i965/fs: Predicate byte scattered writes if needed

While on Untyped Surface messages the bits of the execution mask are
ANDed with the corresponding bits of the Pixel/Sample Mask, that is
not the case for byte scattered writes. That is needed to avoid ssbo
stores writing on helper invocations. So when that can affect, we load
the sample mask, and predicate the send message.

Note: the need for this patch was tested with a custom test. Right now
the 16 bit storage CTS tests doesnt need this path in order to get a
full pass.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>