[Pipeline] Move some GlobalOpt/GlobalDCE runs into simplification pipeline

[aeubanks]

Summary

These are very clearly more simplification than optimization.

Mostly NFC, except for some ordering around passes that don't really matter.

[aeubanks]

comment

[nikic]

LGTM

llvm/lib/Passes/PassBuilderPipelines.cpp

else
     MPM.addPass(buildInlinerPipeline(Level, Phase));
 
   // Remove any dead arguments exposed by cleanups, constant folding globals,
   // and argument promotion.
   MPM.addPass(DeadArgumentEliminationPass());
 
   MPM.addPass(CoroCleanupPass());
 
+  // Optimize globals now that functions are fully simplified.
+  MPM.addPass(GlobalOptPass());
+
+  // Remove dead code, except in the ThinLTO pre-link pipeline where we may want
+  // to keep available_externally functions.
+  if (Phase != ThinOrFullLTOPhase::ThinLTOPreLink)
+    MPM.addPass(GlobalDCEPass());
+
   if (EnableMemProfiler && Phase != ThinOrFullLTOPhase::ThinLTOPreLink) {
     MPM.addPass(createModuleToFunctionPassAdaptor(MemProfilerPass()));
     MPM.addPass(ModuleMemProfilerPass());
   }
 
   return MPM;
 }
 

 
 ModulePassManager
 PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
                                              ThinOrFullLTOPhase LTOPhase) {
   const bool LTOPreLink = (LTOPhase == ThinOrFullLTOPhase::ThinLTOPreLink ||
                            LTOPhase == ThinOrFullLTOPhase::FullLTOPreLink);
   ModulePassManager MPM;
 
-  // Optimize globals now that the module is fully simplified.
-  MPM.addPass(GlobalOptPass());
-  MPM.addPass(GlobalDCEPass());
-
   // Run partial inlining pass to partially inline functions that have
   // large bodies.
   if (RunPartialInlining)
     MPM.addPass(PartialInlinerPass());
 
   // Remove avail extern fns and globals definitions since we aren't compiling
   // an object file for later LTO. For LTO we want to preserve these so they
   // are eligible for inlining at link-time. Note if they are unreferenced they
PassBuilder::buildThinLTOPreLinkDefaultPipeline(OptimizationLevel Level) {
   // FIXME: It isn't clear whether this is really the right place to run this
   // in ThinLTO. Because there is another canonicalization and simplification
   // phase that will run after the thin link, running this here ends up with
   // less information than will be available later and it may grow functions in
   // ways that aren't beneficial.
   if (RunPartialInlining)
     MPM.addPass(PartialInlinerPass());
 
-  // Reduce the size of the IR as much as possible.
-  MPM.addPass(GlobalOptPass());
-
   if (PGOOpt && PGOOpt->PseudoProbeForProfiling &&
       PGOOpt->Action == PGOOptions::SampleUse)
     MPM.addPass(PseudoProbeUpdatePass());
 
   // Handle Optimizer{Early,Last}EPCallbacks added by clang on PreLink. Actual
   // optimization is going to be done in PostLink stage, but clang can't add
   // callbacks there in case of in-process ThinLTO called by linker.
   for (auto &C : OptimizerEarlyEPCallbacks)