i965/vec4: track and use independently each flag channel

i965/vec4: track and use independently each flag channel

vec4_live_variables tracks now each flag channel independently, so
vec4_dead_code_eliminate can update the writemask of null registers,
based on which component are alive at the moment. This would allow
vec4_cmod_propagation to optimize out several movs involving null
registers.

v2: added support to track each flag channel independently at vec4
    live_variables, as v1 assumed that it was already doing it, as
    pointed by Francisco Jerez
v3: general cleaningn after Matt Turner's review

Reviewed-by: Matt Turner <mattst88@gmail.com>