intel/fs/gen12: Workaround unwanted SEND execution due to broken NoMask control flow.

This is a less invasive alternative to the workaround documented in
the hardware spec for GEN:BUG:1407528679, which doesn't involve
disabling structured control flow (it's unlikely that switching to
GOTO/JOIN would have actually fixed the problem anyway).

Under some conditions Gen12 hardware can end up executing a BB with
all channels disabled, which will lead to the execution of any NoMask
instructions in it, even though any execution-masked instructions will
be correctly shot down.  This may break assumptions of some NoMask
SEND messages whose descriptor depends on data generated by live
invocations of the shader.

This avoids the problem by predicating certain instructions on an ANY
horizontal predicate that makes sure that their execution is omitted
when all channels of the program are disabled.  The shader-db impact
of this patch seems to be minimal:

total instructions in shared programs: 17169833 -> 17169913 (0.00%)
instructions in affected programs: 30663 -> 30743 (0.26%)
helped: 0
HURT: 42

total cycles in shared programs: 336966176 -> 336968568 (0.00%)
cycles in affected programs: 2367290 -> 2369682 (0.10%)
helped: 0
HURT: 13

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Cc: 20.0 <mesa-stable@lists.freedesktop.org>

src/intel/compiler/brw_fs.h

@@ -107,6 +107,7 @@ public:
    void setup_cs_payload();
    bool fixup_sends_duplicate_payload();
    void fixup_3src_null_dest();
+   bool fixup_nomask_control_flow();
    void assign_curb_setup();
    void assign_urb_setup();
    void convert_attr_sources_to_hw_regs(fs_inst *inst);