[PATCH] Prevent MDNode's RAUW from introducing a reference to a void Value.

[Frederic Riss]

Seems I was wrong about being able to extend the testcase. Changing the scope of the fix didn't change the fact that I need a constant bitcast inside a call to trigger that code path. And I'm not able to produce this without resorting to UB at the C level. 

Unless you have another idea, I think I'll commit the test I have.

http://reviews.llvm.org/D5828