intel/fs: limit register flag interaction of FIND_*LIVE_CHANNEL

Those instructions do not access the flag registers on Gfx8+. Removing
the interaction enables CSE to remove more of those instructions.

Results are a bit mixed (DG2 vulkan fossils):

  ACO:
  Totals from 127 (5.97% of 2128) affected shaders:
  Instrs: 139966 -> 138972 (-0.71%); split: -0.85%, +0.14%
  Cycles: 1685747 -> 1667480 (-1.08%); split: -2.35%, +1.26%
  Max live registers: 10582 -> 10544 (-0.36%)
  Max dispatch width: 1048 -> 1040 (-0.76%)

  Cyberpunk 2077:
  Totals from 2879 (27.95% of 10301) affected shaders:
  Instrs: 4264789 -> 4225666 (-0.92%); split: -1.01%, +0.09%
  Cycles: 72380209 -> 71619521 (-1.05%); split: -1.63%, +0.58%
  Subgroup size: 30624 -> 30632 (+0.03%)
  Spill count: 98 -> 101 (+3.06%)
  Fill count: 90 -> 93 (+3.33%)
  Scratch Memory Size: 8192 -> 9216 (+12.50%)
  Max live registers: 217807 -> 217098 (-0.33%); split: -0.59%, +0.26%
  Max dispatch width: 23792 -> 24112 (+1.34%)

  Gaining 40 SIMD16 shaders

  Rise Of The Tomb Raider:
  Totals from 622 (5.06% of 12289) affected shaders:
  Instrs: 437380 -> 434760 (-0.60%); split: -0.72%, +0.12%
  Cycles: 261843085 -> 261580703 (-0.10%); split: -0.73%, +0.63%
  Max live registers: 27731 -> 27766 (+0.13%); split: -1.01%, +1.14%
  Max dispatch width: 5832 -> 5432 (-6.86%); split: +0.27%, -7.13%

  Loosing 26 SIMD32 shaders

  Strange Brigade:
  Totals from 1298 (31.48% of 4123) affected shaders:
  Instrs: 1504408 -> 1487968 (-1.09%); split: -1.17%, +0.08%
  Cycles: 20735976 -> 20443216 (-1.41%); split: -1.60%, +0.19%
  Max live registers: 89911 -> 89957 (+0.05%)

DG2 shader-db run:

  total instructions in shared programs: 23130895 -> 23130036 (<.01%)
  instructions in affected programs: 260956 -> 260097 (-0.33%)
  helped: 234
  HURT: 101
  helped stats (abs) min: 1 max: 54 x̄: 6.36 x̃: 4
  helped stats (rel) min: 0.05% max: 8.16% x̄: 2.01% x̃: 1.90%
  HURT stats (abs)   min: 1 max: 37 x̄: 6.23 x̃: 3
  HURT stats (rel)   min: 0.02% max: 5.67% x̄: 0.89% x̃: 0.55%
  95% mean confidence interval for instructions value: -3.62 -1.51
  95% mean confidence interval for instructions %-change: -1.33% -0.94%
  Instructions are helped.

  total loops in shared programs: 6071 -> 6071 (0.00%)
  loops in affected programs: 0 -> 0
  helped: 0
  HURT: 0

  total cycles in shared programs: 898610645 -> 898557166 (<.01%)
  cycles in affected programs: 18308201 -> 18254722 (-0.29%)
  helped: 315
  HURT: 48
  helped stats (abs) min: 1 max: 19312 x̄: 404.23 x̃: 128
  helped stats (rel) min: 0.02% max: 28.98% x̄: 3.92% x̃: 2.65%
  HURT stats (abs)   min: 2 max: 14478 x̄: 1538.60 x̃: 409
  HURT stats (rel)   min: <.01% max: 23.24% x̄: 3.34% x̃: 0.41%
  95% mean confidence interval for cycles value: -333.68 39.03
  95% mean confidence interval for cycles %-change: -3.51% -2.41%
  Inconclusive result (value mean confidence interval includes 0).

  total spills in shared programs: 5964 -> 5964 (0.00%)
  spills in affected programs: 0 -> 0
  helped: 0
  HURT: 0

  total fills in shared programs: 6909 -> 6909 (0.00%)
  fills in affected programs: 0 -> 0
  helped: 0
  HURT: 0

  total sends in shared programs: 1040266 -> 1040266 (0.00%)
  sends in affected programs: 0 -> 0
  helped: 0
  HURT: 0

  LOST:   3
  GAINED: 1

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24553>

src/intel/compiler/brw_fs.cpp

@@ -1012,6 +1012,9 @@ fs_inst::flags_written(const intel_device_info *devinfo) const
    /* On Gfx4 and Gfx5, sel.l (for min) and sel.ge (for max) are implemented
     * using a separate cmpn and sel instruction.  This lowering occurs in
     * fs_vistor::lower_minmax which is called very, very late.
+    *
+    * FIND_LIVE_CHANNEL & FIND_LAST_LIVE_CHANNEL are lowered in
+    * lower_find_live_channel() on Gfx8+ and do not use the flag registers.
     */
    if ((conditional_mod && ((opcode != BRW_OPCODE_SEL || devinfo->ver <= 5) &&
                             opcode != BRW_OPCODE_CSEL &&
@@ -1019,8 +1022,9 @@ fs_inst::flags_written(const intel_device_info *devinfo) const
                             opcode != BRW_OPCODE_WHILE)) ||
        opcode == FS_OPCODE_FB_WRITE) {
       return flag_mask(this, 1);
-   } else if (opcode == SHADER_OPCODE_FIND_LIVE_CHANNEL ||
-              opcode == SHADER_OPCODE_FIND_LAST_LIVE_CHANNEL ||
+   } else if ((devinfo->ver <= 7 &&
+               (opcode == SHADER_OPCODE_FIND_LIVE_CHANNEL ||
+                opcode == SHADER_OPCODE_FIND_LAST_LIVE_CHANNEL)) ||
               opcode == FS_OPCODE_LOAD_LIVE_CHANNELS) {
       return flag_mask(this, 32);
    } else {