i965/fs: Only emit FS_OPCODE_PLACEHOLDER_HALT if there are discards

Based originally on a patch from Ken in May 2014 of the same title. Things
changed enough that I didn't feel comfortable leaving his authorship.

v2: Replace fp->UsesKill with wm_prog_data->uses_kill. Since Ken took the time
to also explain the difference to me, here is his explanation for posterity:

"fp->UsesKill indicates that a ARB_fragment_program shader uses the KIL
instruction, or that a GLSL shader uses the "discard" insntruction
(which are analogous).

On Gen4-5, we sometimes have to simulate OpenGL's "Alpha Test" feature
by emitting shader code that implicitly does a "discard" instruction.

In the key setup, we do:

   /* key->alpha_test_func means simulating alpha testing via discards,
    * so the shader definitely kills pixels.
    */
   prog_data.uses_kill = fp->program.UsesKill || key->alpha_test_func;

Even though the shader may not technically contain a "discard", we need
to act as if it does.

I've also been trying to move the i965 state setup code to use
brw_wm_prog_key for everything, rather than poking at core Mesa's
gl_program/gl_fragment_program/gl_shader/gl_shader_program structures.

--Ken"

Signed-off-by: Ben Widawsky <[email protected]>
Reviewed-by: Matt Turner <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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