i965/fs: Implement opt_sampler_eot() in terms of logical sends.

i965/fs: Implement opt_sampler_eot() in terms of logical sends.

This makes the whole LOAD_PAYLOAD munging unnecessary which simplifies
the code and will allow the optimization to succeed in more cases
independent of whether the LOAD_PAYLOAD instruction can be found or
not.

The following patch is squashed in:

SQUASH: i965/fs: Add basic dataflow check to opt_sampler_eot().

The sampler EOT optimization pass naively assumes that the texturing
instruction provides all the data used by the FB write just because
they're standing next to each other.  The least we should be checking
is whether the source and destination regions of the FB write and
texturing instructions match.  Without this the previous seemingly
harmless patch would have caused opt_sampler_eot() to misoptimize a
shader from dota-2 causing DCE to eliminate all of its 78 instructions
except for the final sampler EOT message (!).

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