[AIX] simplify xcoff part of function TargetMachine::shouldAssumeDSOLocal()

[DiggerLin]

Summary

since the patch "[TargetMachine] Drop implied dso_local for definitions in ELF static relocation" delete some elf related code. some xcoff related can also be simplified after that patch

[hubert.reinterpretcast]

llvm/lib/Target/TargetMachine.cpp

154

I think this looks right, but I'm not sure an extra assert is entirely "NFC".

[hubert.reinterpretcast]

@sfertile, any comments?

[DiggerLin]

since the commit did the same change , I closed the patch

commit c5ee3123685ca5d555f80ed6e1bb8eeefca3dfbf
Author: Fangrui Song <[e-mail]>
Date:   Sat Aug 21 12:37:29 2021 -0700

    [TargetMachine] Simplify shouldAssumeDSOLocal. NFC

llvm/lib/Target/TargetMachine.cpp

154

thanks.

llvm/lib/Target/TargetMachine.cpp

if (TT.isOSBinFormatCOFF() || TT.isOSWindows())
     return true;
 
   if (TT.isOSBinFormatMachO()) {
     if (RM == Reloc::Static)
       return true;
     return GV->isStrongDefinitionForLinker();
   }
 
-  // Due to the AIX linkage model, any global with default visibility is
-  // considered non-local.
-  if (TT.isOSBinFormatXCOFF())
-    return false;
-
-  assert(TT.isOSBinFormatELF() || TT.isOSBinFormatWasm());
+  assert(TT.isOSBinFormatELF() || TT.isOSBinFormatWasm() ||
+         TT.isOSBinFormatXCOFF());
   assert(RM != Reloc::DynamicNoPIC);
   return false;
 }
 
 bool TargetMachine::useEmulatedTLS() const {
   // Returns Options.EmulatedTLS if the -emulated-tls or -no-emulated-tls
   // was specified explicitly; otherwise uses target triple to decide default.
   if (Options.ExplicitEmulatedTLS)
     return Options.EmulatedTLS;