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

[David Blaikie]

Generally looks OK - but I'll leave it to someone more confident with the ValueHandle, RAUW, etc semantics to sign off on.

Ideally I'd probably like a more valid test case as, if we optimize away this scenario at some point the test case might become invalid (either testing parts of instcombine we don't actually ever need it to handle, or maybe instcombine would just eliminate such instructions - in which case this test case would no longer be testing this codepath, for example)

http://reviews.llvm.org/D5828