pan/midgard: Add distance metric to choose_instruction

pan/midgard: Add distance metric to choose_instruction

We require chosen instructions to be "close", to avoid ballooning
register pressure. This is a kludge that will go away once we have
proper liveness tracking in the scheduler, but for now it prevents a lot
of needless spilling.

v2: Lower threshold to 6 (from 8). Schedule is hurt, but a few shaders
that spilled excessively are fixed.

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