i965/vec4/dce: improve track of partial flag register writes

i965/vec4/dce: improve track of partial flag register writes

This is required for correctness in presence of multiple 4-wide flag
writes (e.g. 4-wide instructions with a conditional mod set) which
update a different portion of the same 8-bit flag subregister.

Right now we keep track of flag dataflow with 8-bit granularity and
consider flag writes to have killed any previous definition of the
same subregister even if the write was less than 8 channels wide,
which can cause live flag register updates to be dead
code-eliminated incorrectly.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>