[PBQP] Tidy up CostAllocator.h: fix variable case, rename CostPool to ValuePool.

No functional change. This just brings things more in-line with coding
standards, and makes ValuePool's functionality clearer (it's not tied to pooling
costs, and we may want to use it to hold other things in the future).



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220641 91177308-0d34-0410-b5e6-96231b3b80d8