[Debugify] Don't apply DI before the bitcode writer pass

Applying synthetic debug info before the bitcode writer pass has no
testing-related purpose. This commit prevents that from happening.

It also adds tests which check that IR produced with/without
-debugify-each enabled is identical after stripping. This makes it
possible to check that individual passes (or full pipelines) are
invariant to debug info.

llvm-svn: 333861

lib/Bitcode/Writer/BitcodeWriterPass.cpp

@@ -80,3 +80,7 @@ ModulePass *llvm::createBitcodeWriterPass(raw_ostream &Str,
   return new WriteBitcodePass(Str, ShouldPreserveUseListOrder,
                               EmitSummaryIndex, EmitModuleHash);
 }
+
+bool llvm::isBitcodeWriterPass(Pass *P) {
+  return P->getPassID() == (llvm::AnalysisID)&WriteBitcodePass::ID;
+}