Bitcode: Simplify emission of METADATA_BLOCK

Refactor logic so that we know up-front whether to open a block and
whether we need an MDString abbreviation.

This is almost NFC, but will start emitting `MDString` abbreviations
when the first record is not an `MDString`.

llvm-svn: 225712

llvm/lib/Bitcode/Writer/ValueEnumerator.cpp

@@ -282,7 +282,7 @@ static bool isIntOrIntVectorValue(const std::pair<const Value*, unsigned> &V) {
   return V.first->getType()->isIntOrIntVectorTy();
 }
 
-ValueEnumerator::ValueEnumerator(const Module &M) {
+ValueEnumerator::ValueEnumerator(const Module &M) : HasMDString(false) {
   if (shouldPreserveBitcodeUseListOrder())
     UseListOrders = predictUseListOrder(M);
 
@@ -546,6 +546,8 @@ void ValueEnumerator::EnumerateMetadata(const Metadata *MD) {
   else if (auto *C = dyn_cast<ConstantAsMetadata>(MD))
     EnumerateValue(C->getValue());
 
+  HasMDString |= isa<MDString>(MD);
+
   // Replace the dummy ID inserted above with the correct one.  MDValueMap may
   // have changed by inserting operands, so we need a fresh lookup here.
   MDs.push_back(MD);