For now the allocator still uses the old (internal) construction mechanism by default. This will be phased out soon assuming
no issues with the builder system come up.
To invoke the new construction mechanism just pass '-regalloc=pbqp -pbqp-builder' to llc. To provide custom constraints a
Target just needs to extend PBQPBuilder and pass an instance of their derived builder to the RegAllocPBQP constructor.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114272 91177308-0d34-0410-b5e6-96231b3b80d8
