intel/fs/gen7+: Swap sample mask flag register and FIND_LIVE_CHANNEL temporary.

FIND_LIVE_CHANNEL was using f1.0-f1.1 as temporary flag register on
Gen7, instead use f0.0-f0.1.  In order to avoid collision with the
discard sample mask, move the latter to f1.0-f1.1.  This makes room
for keeping track of the sample mask of the second half of SIMD32
programs that use discard.

Note that some MOVs of the sample mask into f1.0 become redundant now
in lower_surface_logical_send() and lower_a64_logical_send().

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>x

src/intel/compiler/brw_fs.h

@@ -419,13 +419,15 @@ private:
 
 /**
  * Return the flag register used in fragment shaders to keep track of live
- * samples.
+ * samples.  On Gen7+ we use f1.0-f1.1 to allow discard jumps in SIMD32
+ * dispatch mode, while earlier generations are constrained to f0.1, which
+ * limits the dispatch width to SIMD16 for fragment shaders that use discard.
  */
 static inline unsigned
 sample_mask_flag_subreg(const fs_visitor *shader)
 {
    assert(shader->stage == MESA_SHADER_FRAGMENT);
-   return 1;
+   return shader->devinfo->gen >= 7 ? 2 : 1;
 }
 
 /**