[PBQP] Unique allowed-sets for nodes in the PBQP graph and use pairs of these

sets as keys into a cache of interference matrice values in the Interference
constraint adder.

Creating interference matrices was one of the large remaining time-sinks in
PBQP. Caching them reduces the total compile time (when using PBQP) on the
nightly test suite by ~10%.

llvm-svn: 220688