[CodeGen] Filter out available_externally aliases

The Language Reference says that aliases can have available_externally
linkage if their aliasee is an available_externally global value. Using
this kind of aliases resulted in crashes during code generation, filter
them out (the same that the AsmPrinter also filters out GlobalVariables
in emitSpecialLLVMGlobal(); Functions are discarded in the machine pass
infrastructure).

Differential Revision: https://reviews.llvm.org/D142352

(cherry picked from commit f1c4f92)

Author: hahnjo

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -2235,6 +2235,8 @@ bool AsmPrinter::doFinalization(Module &M) {
   SmallVector<const GlobalAlias *, 16> AliasStack;
   SmallPtrSet<const GlobalAlias *, 16> AliasVisited;
   for (const auto &Alias : M.aliases()) {
+    if (Alias.hasAvailableExternallyLinkage())
+      continue;
     for (const GlobalAlias *Cur = &Alias; Cur;
          Cur = dyn_cast<GlobalAlias>(Cur->getAliasee())) {
       if (!AliasVisited.insert(Cur).second)

llvm/test/CodeGen/Generic/available_externally_alias.ll

@@ -0,0 +1,11 @@
+; RUN: llc < %s
+
+@v = available_externally global i32 42, align 4
+@va = available_externally alias i32, ptr @v
+
+define available_externally i32 @f() {
+entry:
+  ret i32 0
+}
+
+@fa = available_externally alias i32(), ptr @f