freedreno/ir3: add support for a6xx 'merged' register set

freedreno/ir3: add support for a6xx 'merged' register set

Starting with a6xx, half and full precision registers conflict.  Which
makes things a bit more efficient, ie. if some parts of the shader are
heavy on half-precision and others on full precision, you don't have to
allocate the worst case for both.  But it means we need to setup some
additional conflicts.

Signed-off-by: Rob Clark <robdclark@gmail.com>