i965: Print instructions' children during scheduling debugging.

i965: Print instructions' children during scheduling debugging.

Useful for tracking down problems in dependency calculations.

Scheduling debugging now prints:

clock    2, scheduled: linterp vgrf5, hw_reg2, hw_reg3, hw_reg0,
        child 0, 53 parents: fb_write (null), (null), (null), (null),
        child 1, 2 parents: tex vgrf4, vgrf5, (null), (null),
        child 2, 52 parents: placeholder_halt (null), (null), (null), (null),
clock    4, scheduled: linterp vgrf5+1, hw_reg2, hw_reg3, hw_reg0+16,
        child 0, 52 parents: fb_write (null), (null), (null), (null),
        child 1, 1 parents: tex vgrf4, vgrf5, (null), (null),
                now available
        child 2, 51 parents: placeholder_halt (null), (null), (null), (null),

Reviewed-by: Eric Anholt <eric@anholt.net>