r600g/sb: fix handling of interference sets in post_scheduler

r600g/sb: fix handling of interference sets in post_scheduler

post_scheduler clears interference set for reallocatable values when
the value becomes live first time, and then updates it to take into
account modified order of operations, but this was not handled properly
if the value appears first time as a source in copy operation.

Fixes issues with webgl demo: http://madebyevan.com/webgl-water/

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>