freedreno/ir3: make mergedregs a property of the variant

freedreno/ir3: make mergedregs a property of the variant

Rather than assuming a6xx+ means mergedregs.  We can actually (mostly?)
do splitregs on a6xx as well.  And GS/DS/HS currently require it, which
might be papering over a bug, or might be something to do with how
chaining shaders works.  At any rate, we should at least be consistent,
and not have the compiler thinking we are doing mergedregs when we are
actually doing splitregs.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5458>