Added a separate class (PBQPBuilder) for PBQP Problem construction. This class can be extended to support custom constraints.

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.

llvm-svn: 114272