i965: Compute the node's delay time for scheduling.

i965: Compute the node's delay time for scheduling.

This is a step in doing scheduling as described in Muchnick (p538).  A
difference is that our latency function is only specific to one
instruction (it doesn't describe, for example, the different latency
between WAR of a send's arguments and RAW of a send's destination), but
that's changeable later.  We also don't separately compute the postorder
traversal of the graph, since we can use the setting of the delay field as
the "visited" flag.

Reviewed-by: Paul Berry <stereotype441@gmail.com>