BitcodeReader: Fix non-determinism in use-list order

`BasicBlockFwdRefs` (and `BlockAddrFwdRefs` before it) was being emptied
in a non-deterministic order.  When predicting use-list order I've
worked around this another way, but even when parsing lazily (and we
can't recreate use-list order) use-lists should be deterministic.

Make them so by using a side-queue of functions with forward-referenced
blocks that gets visited in order.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214899 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/BitcodeReader.h

@@ -22,6 +22,7 @@
 #include "llvm/IR/OperandTraits.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/ValueHandle.h"
+#include <deque>
 #include <system_error>
 #include <vector>
 
@@ -183,6 +184,7 @@ class BitcodeReader : public GVMaterializer {
   /// inserted lazily into functions when they're loaded.
   typedef std::pair<unsigned, BasicBlock *> BasicBlockRefTy;
   DenseMap<Function *, std::vector<BasicBlockRefTy>> BasicBlockFwdRefs;
+  std::deque<Function *> BasicBlockFwdRefQueue;
 
   /// UseRelativeIDs - Indicates that we are using a new encoding for
   /// instruction operands where most operands in the current