st/mesa: reduce time spent in calculating temp read/writes

st/mesa: reduce time spent in calculating temp read/writes

The glsl->tgsi convertor does some temporary register reduction
however in profiling shader-db this shows up quite highly,

so optimise things to reduce the number of loops through
all the instructions we do. This drops merge_registers
from 4-5% on the profile to 1%. I think this can be reduced
further by possibly optimising the renumber pass.

Acked-by: Marek Olšák <marek.olsak@amd.com
Signed-off-by: Dave Airlie <airlied@redhat.com>