[Statepoints 2/4] Statepoint infrastructure for garbage collection: MI & x86-64 Backend

This is the second patch in a small series.  This patch contains the MachineInstruction and x86-64 backend pieces required to lower Statepoints.  It does not include the code to actually generate the STATEPOINT machine instruction and as a result, the entire patch is currently dead code.  I will be submitting the SelectionDAG parts within the next 24-48 hours.  Since those pieces are by far the most complicated, I wanted to minimize the size of that patch.  That patch will include the tests which exercise the functionality in this patch.  The entire series can be seen as one combined whole in http://reviews.llvm.org/D5683.

The STATEPOINT psuedo node is generated after all gc values are explicitly spilled to stack slots.  The purpose of this node is to wrap an actual call instruction while recording the spill locations of the meta arguments used for garbage collection and other purposes.  The STATEPOINT is modeled as modifing all of those locations to prevent backend optimizations from forwarding the value from before the STATEPOINT to after the STATEPOINT.  (Doing so would break relocation semantics for collectors which wish to relocate roots.)

The implementation of STATEPOINT is closely modeled on PATCHPOINT.  Eventually, much of the code in this patch will be removed.  The long term plan is to merge the functionality provided by statepoints and patchpoints.  Merging their implementations in the backend is likely to be a good starting point.

Reviewed by: atrick, ributzka