Moved the PBQP allocator class out of the header and back in to the cpp file...

Moved the PBQP allocator class out of the header and back in to the cpp file to hide the gory details.
Allocator instances can now be created by calling createPBQPRegisterAllocator.

Tidied up use of CoalescerPair as per Jakob's suggestions.

Made the new PBQPBuilder based construction process the default. The internal construction process
remains in-place and available via -pbqp-builder=false for now. It will be removed shortly if the new
process doesn't cause any regressions.

llvm-svn: 114626