[lto] Enable new PM when the PM config is non-empty

This restores the behavior before 964f8103c5, which broke 2 tests:
  LLVM :: tools/llvm-lto2/X86/pipeline.ll
  lld :: ELF/lto/ltopasses-custom.ll

llvm/lib/LTO/LTOBackend.cpp

@@ -355,7 +355,7 @@ bool lto::opt(const Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
                                /*Cmdline*/ CmdArgs);
   }
   // FIXME: Plumb the combined index into the new pass manager.
-  if (Conf.UseNewPM) {
+  if (Conf.UseNewPM || !Conf.OptPipeline.empty()) {
     runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
                    ImportSummary);
   } else {