[GlobalOpt] Never remove global values from non-discardable COMDATs

[Aaron1011]

Summary

Currently, GlobalOpt will remove global values from non-discardable COMDATS (COMDATS which have at least one live object) if the value in question has local linkage.
As described in https://reviews.llvm.org/D53234, this is an invalid treatment of COMDAT members, and can lead to issues when using MD_Associated metadata.

I've modified GlobalOpt to always bail out of global value removal if the global value is in a non-discardable COMDAT.
I've also modified the associated test to check that one of its functions, which GlobalOpt currently removes due to its local linkage, is now kept.

[Aaron1011]

Ping

lib/Transforms/IPO/GlobalOpt.cpp

 static bool deleteIfDead(
     GlobalValue &GV, SmallPtrSetImpl<const Comdat *> &NotDiscardableComdats) {
   GV.removeDeadConstantUsers();
 
   if (!GV.isDiscardableIfUnused() && !GV.isDeclaration())
     return false;
 
   if (const Comdat *C = GV.getComdat())
-    if (!GV.hasLocalLinkage() && NotDiscardableComdats.count(C))
+    if (NotDiscardableComdats.count(C))
       return false;
 
   bool Dead;
   if (auto *F = dyn_cast<Function>(&GV))
     Dead = (F->isDeclaration() && F->use_empty()) || F->isDefTriviallyDead();
   else
     Dead = GV.use_empty();
   if (!Dead)

test/Transforms/GlobalOpt/preserve-comdats.ll

define void @init_comdat_global() {
   ret void
 }
 ; CHECK: define void @init_comdat_global()
 
 define internal void @init_simple_global() comdat($comdat_global) {
   store i8 42, i8* @simple_global
   ret void
 }
-; CHECK-NOT: @init_simple_global()
+; CHECK: define internal fastcc void @init_simple_global() unnamed_addr comdat($comdat_global)
 
 define i8* @use_simple() {
   ret i8* @simple_global
 }
 ; CHECK: define i8* @use_simple()
 
 define i8* @use_comdat() {
   ret i8* @comdat_global
 }
 ; CHECK: define i8* @use_comdat()