[PBQP] Fix coalescing benefits

As coalescing registers is a benefit, the cost should be improved (i.e. made smaller) when coalescing is possible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220302 91177308-0d34-0410-b5e6-96231b3b80d8
[PBQP] Check for out of bound access in DEBUG builds

It is just too easy to use a virtual register intead of a NodeId without a
compiler warning. This does not fix the fundamental problem, i.e. both
have the same underlying types, but increases the likelyhood to detect it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220303 91177308-0d34-0410-b5e6-96231b3b80d8