[PATCH] Make variable argument intrinsics behave correctly in a Win64 CC function.

[Charles Davis]

  @resistor wrote:
  > The generic SelectionDAG changes look fine to me in principle. I'm not familiar with the X86 or MS-specific lowering.

  I know. I just wanted to make sure the `SelectionDAG` changes were fine.

http://llvm-reviews.chandlerc.com/D1622