[MachinePipeliner] Refine the RecMII calculation

In the case of more than one SDep  between two successor SUnits in the Nodeset, the current implementation sums the latencies of the dependencies, which could create a larger RecMII than necessary.

for example, in case there is both a data dependency and an output dependency (with latency > 0) between successor nodes:
SU(1) inst1:
  successors:
    SU(2): out  latency = 1
    SU(2): data latency = 1
SU(2) inst2:
  successors:
    SU(3): out  latency = 1
    SU(3): data latency = 1
SU(3) inst3:
  successors:
    SU(1): out  latency = 1
    SU(1): data latency = 1

the NodeSet latency returned would be 6, whereas it could be 3 if we take the max for each successor SUnit.
In general this can be extended to finding the shortest path in the recurrence..
thoughts?

Unfortunately I had a hard time creating a test for this in Hexagon/PowerPC, so help would be appreciated.

Reviewed By: bcahoon

Differential Revision: https://reviews.llvm.org/D75918