intel/brw: Mark FIND[_LAST]_LIVE_CHANNEL as not writing the flag

brw_lower_find_live_channel doesn't actually write a flag register, but elk_find_live_channel notes that the flag was used on Gfx7.

This allows more CSE on FIND[_LAST]_LIVE_CHANNEL.

shader-db and fossil-db on Alchemist show minor reductions in cycles and instruction count, a few minor increases, but it doesn't seem to be a large effect in either direction.