MI-Sched: added tracking of dependent latency for better heuristics.

Heuristics compare the critical path in the scheduled code, called
ExpectedLatency, with the latency of instructions remaining to be
scheduled. There are two ways to look at remaining latency:

(1) Dependent latency includes the latency between unscheduled and
scheduled instructions.

(2) Independent latency is simply the height (bottom-up) or depth
(top-down) of instructions currently in the ready Q.

llvm-svn: 184029