[StructuralHash] Ignore global variable declarations

Ignore declarations of global variables, just as we do with declarations
of functions.

Done as a follow up to the comments in https://reviews.llvm.org/D149209

Differential Revision: https://reviews.llvm.org/D153855

# Conflicts:
#	llvm/lib/IR/StructuralHash.cpp

llvm/lib/IR/StructuralHash.cpp

@@ -58,9 +58,10 @@ class StructuralHashImpl {
   }
 
   void update(const GlobalVariable &GV) {
-    // used/compiler.used don't affect analyses.
+    // Declarations and used/compiler.used don't affect analyses.
     // Same for llvm.embedded.object, which is always a metadata section.
-    if (GV.getName() == "llvm.compiler.used" || GV.getName() == "llvm.used" ||
+    if (GV.isDeclaration() ||
+        GV.getName() == "llvm.compiler.used" || GV.getName() == "llvm.used" ||
         GV.getName() == "llvm.embedded.object")
       return;
     hash(23456); // Global header

llvm/test/Transforms/StripDeadPrototypes/remove-global-variable-declaration.ll

@@ -0,0 +1,9 @@
+; RUN: opt -passes=strip-dead-prototypes -S -verify-analysis-invalidation < %s | FileCheck %s
+
+; The declaration of the unused global variable @.str should be removed without
+; getting any error from -verify-analysis-invalidation.
+
+; CHECK-NOT: @.str
+
+@.str = external constant [15 x i16]
+