Refactor duplicated check for valid declaration linkage. NFC.

llvm-svn: 269184
diff --git a/llvm/lib/AsmParser/LLParser.cpp b/llvm/lib/AsmParser/LLParser.cpp
index 4994288..ea04eb7 100644
--- a/llvm/lib/AsmParser/LLParser.cpp
+++ b/llvm/lib/AsmParser/LLParser.cpp
@@ -825,8 +825,9 @@
   // If the linkage is specified and is external, then no initializer is
   // present.
   Constant *Init = nullptr;
-  if (!HasLinkage || (Linkage != GlobalValue::ExternalWeakLinkage &&
-                      Linkage != GlobalValue::ExternalLinkage)) {
+  if (!HasLinkage ||
+      !GlobalValue::isValidDeclarationLinkage(
+          (GlobalValue::LinkageTypes)Linkage)) {
     if (ParseGlobalValue(Ty, Init))
       return true;
   }