[PM] BitcodeWriterPass should derive from PassInfoMixin

[tejohnson]

Summary

The BitcodeWriterPass was ported a couple years ago, and predates the
PassInfoMixin. Make BitcodeWriterPass from that base class.

Should BitcodeWriterPass be added to the PassRegistry.def file? It seems
like that is only for passes that can be added arbitrarily, e.g. via the
-passes flag to the opt tool. Whereas the bitcode writer is added
specially based on the output type (and requires an output stream and
other parameters). For now I have left it out of the PassRegistry, but
let me know if it should go there.

Finally, I was considering an NFC change of the legacy WriteBitcodePass
to BitcodeWriterLegacyPass to make its usage clearer and more consistent
with other legacy passes. WDYT?

[mehdi_amini]

include/llvm/Bitcode/BitcodeWriterPass.h

70–71

The only point of the Mixin right now is to avoid this method, so you should probably remove it as well.

[tejohnson]

Remove name() method, which is not needed when deriving from Mixin

[mehdi_amini]

LGTM.
I'm not sure about the registration, I think right now it is only to be able to schedule pass from a string? This shouldn't apply this this pass.

TBH, I even wonder if having the bitcode writer as a pass is useful at all? (It does not really hurt keeping it though...)

[tejohnson]

In D23465#514339, @mehdi_amini wrote:

LGTM.
I'm not sure about the registration, I think right now it is only to be able to schedule pass from a string? This shouldn't apply this this pass.

Yeah, that's my thought. Will commit as-is, if others think it needs to be registered that can be added as a follow-on.

include/llvm/Bitcode/BitcodeWriterPass.h

ModulePass *createBitcodeWriterPass(raw_ostream &Str,
                                     bool ShouldPreserveUseListOrder = false,
                                     bool EmitSummaryIndex = false,
                                     bool EmitModuleHash = false);
 
 /// \brief Pass for writing a module of IR out to a bitcode file.
 ///
 /// Note that this is intended for use with the new pass manager. To construct
 /// a pass for the legacy pass manager, use the function above.
-class BitcodeWriterPass {
+class BitcodeWriterPass : public PassInfoMixin<BitcodeWriterPass> {
   raw_ostream &OS;
   bool ShouldPreserveUseListOrder;
   bool EmitSummaryIndex;
   bool EmitModuleHash;
 
 public:
   /// \brief Construct a bitcode writer pass around a particular output stream.
   ///
   /// If \c ShouldPreserveUseListOrder, encode use-list order so it can be
   /// reproduced when deserialized.
   ///
   /// If \c EmitSummaryIndex, emit the summary index (currently
   /// for use in ThinLTO optimization).
   explicit BitcodeWriterPass(raw_ostream &OS,
                              bool ShouldPreserveUseListOrder = false,
                              bool EmitSummaryIndex = false,
                              bool EmitModuleHash = false)
       : OS(OS), ShouldPreserveUseListOrder(ShouldPreserveUseListOrder),
   EmitSummaryIndex(EmitSummaryIndex), EmitModuleHash(EmitModuleHash) {}
 
   /// \brief Run the bitcode writer pass, and output the module to the selected
   /// output stream.
   PreservedAnalyses run(Module &M, ModuleAnalysisManager &);
-
-  static StringRef name() { return "BitcodeWriterPass"; }
 };
 
 }
 
 #endif