Don't crash when llvm.compiler.used becomes empty.

GlobalOpt simplifies llvm.compiler.used by removing any members that are also
in the more strict llvm.used. Handle the special case where llvm.compiler.used
becomes empty.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186778 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/IPO/GlobalOpt.cpp b/lib/Transforms/IPO/GlobalOpt.cpp
index 669162c..183a599 100644
--- a/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/lib/Transforms/IPO/GlobalOpt.cpp
@@ -3070,6 +3070,11 @@
 
 static void setUsedInitializer(GlobalVariable &V,
                                SmallPtrSet<GlobalValue *, 8> Init) {
+  if (Init.empty()) {
+    V.eraseFromParent();
+    return;
+  }
+
   SmallVector<llvm::Constant *, 8> UsedArray;
   PointerType *Int8PtrTy = Type::getInt8PtrTy(V.getContext());