rdar://13126763

Fix a bug in DAGCombine. The symptom is mistakenly optimizing expression
"x + x*x" into "x * 3.0".
This patch makes "&Cls::purevfn" not an odr use. This isn't what the standard
says, but that's a defect (to be filed). "Cls::purevfn()" is still an odr use.

Also fixes a bug in the previous patch that caused us to not mark the function
referenced just because we didn't want to mark it odr used.
Revert accidental commit (ran svn commit from wrong directory).
This patch makes "&Cls::purevfn" not an odr use. This isn't what the standard
says, but that's a defect (to be filed). "Cls::purevfn()" is still an odr use.

Also fixes a bug that caused us to not mark the function referenced just
because we didn't want to mark it odr used.