Allow basic blocks in metadata [NFC]

[jdoerfert]

Hey Duncan,

Git blame named you as owner of this part of the Metadata.cpp. Could you take a lookt at this minimal change we need for our in-house vectorizer. Becasue we also distribute it as a loadable extension for pre-compiled LLVM we would like to get it in mainline [hopefully 3.8.x].

Cheers,
Johannes

[dexonsmith]

Code change looks good, but please add a unit test that exposes this to:

unittests/IR/MetadataTest.cpp

lib/IR/Metadata.cpp

 }
 
 ValueAsMetadata *ValueAsMetadata::get(Value *V) {
   assert(V && "Unexpected null Value");
 
   auto &Context = V->getContext();
   auto *&Entry = Context.pImpl->ValuesAsMetadata[V];
   if (!Entry) {
-    assert((isa<Constant>(V) || isa<Argument>(V) || isa<Instruction>(V)) &&
+    assert((isa<Constant>(V) || isa<Argument>(V) || isa<Instruction>(V) ||
+            isa<BasicBlock>(V)) &&
            "Expected constant or function-local value");
     assert(!V->IsUsedByMD &&
            "Expected this to be the only metadata use");
     V->IsUsedByMD = true;
     if (auto *C = dyn_cast<Constant>(V))
       Entry = new ConstantAsMetadata(C);
     else
       Entry = new LocalAsMetadata(V);