AMDGPU: Don't run redundant GlobalDCE

[arsenm]

Summary

The internalize pass is added early, and
GlobalOpt / GlobalDCE are already run later.

[rampitec]

It is not redundant. It is here to get rid of unused library functions before we start optimizing them.

lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

[Internalize, EarlyInline, AMDGPUAA](const PassManagerBuilder &,
                                          legacy::PassManagerBase &PM) {
       if (AMDGPUAA) {
         PM.add(createAMDGPUAAWrapperPass());
         PM.add(createAMDGPUExternalAAWrapperPass());
       }
       PM.add(createAMDGPUUnifyMetadataPass());
       if (Internalize) {
         PM.add(createInternalizePass(mustPreserveGV));
-        PM.add(createGlobalDCEPass());
       }
       if (EarlyInline)
         PM.add(createAMDGPUAlwaysInlinePass(false));
   });
 
   Builder.addExtension(
     PassManagerBuilder::EP_EarlyAsPossible,
     [AMDGPUAA, LibCallSimplify](const PassManagerBuilder &,