[ThinLTO] Fix assertions on lazy-loading of Metadata TBAA attachments

Summary:
The issue happens with:

 %0 = ....., !tbaa !0
 %1 = ....., !tbaa !1

With !0 that references !1.

In this case when loading !0 we generates a temporary for the
operand !1. We now flush it immediately and trigger the load of
!1 before moving on. If we don't we get the temporary when
attaching to %1. This is usually not an issue except that we
eagerly try to update TBAA MDNodes, which is obviously not possible
if we only have a temporary.

Differential Revision: https://reviews.llvm.org/D28423

llvm-svn: 291362

llvm/lib/Bitcode/Reader/MetadataLoader.cpp

@@ -1562,8 +1562,6 @@ Error MetadataLoader::MetadataLoaderImpl::parseMetadataAttachment(
 
   SmallVector<uint64_t, 64> Record;
 
-  PlaceholderQueue Placeholders;
-
   while (true) {
     BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
 
@@ -1572,7 +1570,6 @@ Error MetadataLoader::MetadataLoaderImpl::parseMetadataAttachment(
     case BitstreamEntry::Error:
       return error("Malformed block");
     case BitstreamEntry::EndBlock:
-      resolveForwardRefsAndPlaceholders(Placeholders);
       return Error::success();
     case BitstreamEntry::Record:
       // The interesting case.
@@ -1608,10 +1605,13 @@ Error MetadataLoader::MetadataLoaderImpl::parseMetadataAttachment(
 
         auto Idx = Record[i + 1];
         if (Idx < (MDStringRef.size() + GlobalMetadataBitPosIndex.size()) &&
-            !MetadataList.lookup(Idx))
+            !MetadataList.lookup(Idx)) {
           // Load the attachment if it is in the lazy-loadable range and hasn't
           // been loaded yet.
+          PlaceholderQueue Placeholders;
           lazyLoadOneMetadata(Idx, Placeholders);
+          resolveForwardRefsAndPlaceholders(Placeholders);
+        }
 
         Metadata *Node = MetadataList.getMetadataFwdRef(Idx);
         if (isa<LocalAsMetadata>(Node))