[PATCH] Introduce an example statepoint GC strategy

[Philip Reames]

Hi whitequark, sanjoy,

This change includes the most basic possible GCStrategy for a GC which is using the statepoint lowering code.  At the moment, this GCStrategy doesn't really do much - aside from actually generate correct stackmaps that is - but I went ahead and added a few extra correctness checks as proof of concept.  It's mostly here to provide documentation on how to do one, and to provide a point for various optimization legality hooks I'd like to add going forward.  (For context, see the TODOs in InstCombine around gc.relocate.)  

Note that I am deliberately not making a GCStrategy required to use gc.statepoints with this change.  I want to give folks out of tree - including myself - a chance to migrate.  In a week or two, I'll make having a GCStrategy be required for gc.statepoints.  To this end, I added the gc tag to one of the test cases but not others.

http://reviews.llvm.org/D6808

Files:
  include/llvm/CodeGen/GCStrategy.h
  include/llvm/CodeGen/GCs.h
  include/llvm/CodeGen/LinkAllCodegenComponents.h
  lib/CodeGen/AsmPrinter/AsmPrinter.cpp
  lib/CodeGen/CMakeLists.txt
  lib/CodeGen/GCStrategy.cpp
  lib/CodeGen/SelectionDAG/StatepointLowering.cpp
  test/CodeGen/X86/statepoint-forward.ll

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