Bring back "Assert that we have all use/users in the getters."

This reverts commit r257751, bringing back r256105.

The problem the assert found was fixed in r257915.

Original commit message:

Assert that we have all use/users in the getters.

An error that is pretty easy to make is to use the lazy bitcode reader
and then do something like

if (V.use_empty())

The problem is that uses in unmaterialized functions are not accounted
for.

This patch adds asserts that all uses are known.

llvm-svn: 257920

llvm/lib/IR/Module.cpp

@@ -394,10 +394,8 @@ std::error_code Module::materialize(GlobalValue *GV) {
 std::error_code Module::materializeAll() {
   if (!Materializer)
     return std::error_code();
-  if (std::error_code EC = Materializer->materializeModule())
-    return EC;
-  Materializer.reset();
-  return std::error_code();
+  std::unique_ptr<GVMaterializer> M = std::move(Materializer);
+  return M->materializeModule();
 }
 
 std::error_code Module::materializeMetadata() {