IR: Don't track nullptr on metadata RAUW

The RAUW support in `Metadata` supports going to `nullptr` specifically
to handle values being deleted, causing `ValueAsMetadata` to be deleted.

Fix the case where the reference is from a `TrackingMDRef` (as opposed
to an `MDOperand` or a `MetadataAsValue`).

This is surprisingly rare -- metadata tracked by `TrackingMDRef` going
to null -- but it came up in an openSUSE bootstrap during inlining.  The
tracking ref was held by the `ValueMap` because it was referencing a
local, the basic block containing the local became dead after it had
been merged in, and when the local was deleted, the tracking ref
asserted in an `isa`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224146 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Metadata.cpp

@@ -173,7 +173,8 @@ void ReplaceableMetadataImpl::replaceAllUsesWith(Metadata *MD) {
       // Update unowned tracking references directly.
       Metadata *&Ref = *static_cast<Metadata **>(Pair.first);
       Ref = MD;
-      MetadataTracking::track(Ref);
+      if (MD)
+        MetadataTracking::track(Ref);
       UseMap.erase(Pair.first);
       continue;
     }