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_nir.cpp

@@ -3490,9 +3490,9 @@ fs_visitor::nir_emit_fs_intrinsic(const fs_builder &bld,
    case nir_intrinsic_discard:
    case nir_intrinsic_demote_if:
    case nir_intrinsic_discard_if: {
-      /* We track our discarded pixels in f0.1.  By predicating on it, we can
-       * update just the flag bits that aren't yet discarded.  If there's no
-       * condition, we emit a CMP of g0 != g0, so all currently executing
+      /* We track our discarded pixels in f0.1/f1.0.  By predicating on it, we
+       * can update just the flag bits that aren't yet discarded.  If there's
+       * no condition, we emit a CMP of g0 != g0, so all currently executing
        * channels will get turned off.
        */
       fs_inst *cmp = NULL;