[PATCH] Fix illegal DAG produced by SelectionDAG::getConstant() for v2i64 type

[Daniel Sanders]

  I believe I've covered both issues that were raised as of the last update to the patch. Although as noted in my earlier comment, I have some concerns that the new state in the SelectionDAG class might not be the right way to prevent SelectionDAG::getConstant() from legalizing too early.

  Could you review the new version?

  This patch depends on the following patches that are currently under review:
  * {D1374}
  * {D1971}
  * {D1972}

http://llvm-reviews.chandlerc.com/D1973