[PBQP] Use DenseSet rather than std::set for PBQP's PoolCostAllocator
implementation.

This is good for a ~6% reduction in total compile time on the nightly test suite
when running with -regalloc=pbqp.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220183 91177308-0d34-0410-b5e6-96231b3b80d8