i965/fs: Keep track of flag dependencies with byte granularity during scheduling.

i965/fs: Keep track of flag dependencies with byte granularity during scheduling.

This prevents false dependencies from being created between
instructions that write disjoint 8-bit portions of the flag register
and OTOH should make sure that the scheduler considers dependencies
between instructions that write or read multiple flag subregisters
at once (e.g. 32-wide predication or conditional mods).

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>