Add MapVector::rbegin(), MapVector::rend() to completment MapVector::begin(), MapVector::end().

These just delegate to the underlying vector type in the MapVector.

Also just add in some sanity unittests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220687 91177308-0d34-0410-b5e6-96231b3b80d8
[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%.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220688 91177308-0d34-0410-b5e6-96231b3b80d8