intel/fs/gen12: Workaround data coherency issues due to broken NoMask control flow.

intel/fs/gen12: Workaround data coherency issues due to broken NoMask control flow.

Together with the fixup_nomask_control_flow() pass introduced in a
previous patch, this implements a less invasive alternative to the
workaround documented in the hardware spec for GEN:BUG:1407528679,
which doesn't involve disabling structured control flow.

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 could break assumptions of the SWSB pass
if the data computed by a NoMask instruction is synchronized against
by using an SWSB annotation baked into a regular execution-masked
instruction, since the first (NoMask) instruction may be executed
redundantly by the hardware, even though the second will correctly be
shot down, potentially leading to a RaW or WaW hazard if a third
instruction subsequently accesses the destination register of the
first instruction.

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