Add a GlobalAlias::isValidLinkage to reduce code duplication.

Thanks to Reid Kleckner for the suggestion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192298 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/IR/Verifier.cpp b/lib/IR/Verifier.cpp
index c010839..c7559c1 100644
--- a/lib/IR/Verifier.cpp
+++ b/lib/IR/Verifier.cpp
@@ -529,8 +529,7 @@
 void Verifier::visitGlobalAlias(GlobalAlias &GA) {
   Assert1(!GA.getName().empty(),
           "Alias name cannot be empty!", &GA);
-  Assert1(GA.hasExternalLinkage() || GA.hasLocalLinkage() ||
-          GA.hasWeakLinkage() || GA.hasLinkOnceLinkage(),
+  Assert1(GlobalAlias::isValidLinkage(GA.getLinkage()),
           "Alias should have external or external weak linkage!", &GA);
   Assert1(GA.getAliasee(),
           "Aliasee cannot be NULL!", &GA);