[xla] Add a method to express that we want to schedule a node as early as
[xla] Add a method to express that we want to schedule a node as early as
possible.

We have kForceDelay to express that we want to schedule a node as late as
possible. This change adds kForceEarly for similar purpose.

This is used for the GPU scheduler initially, for scheduling pipelined Recv
close to RecvDone, so that copies of RecvDone can be removed.
