pan/midgard: Handle fragment writeout in RA

pan/midgard: Handle fragment writeout in RA

Rather than using a pile of hacks and awkward constructs in MIR to
ensure the writeout parameter gets written into r0, let's add a
dedicated shadow register class for writeout (interfering with work
register r0) so we can express the writeout condition succintly and
directly.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>