[JumpThreading] Make test more robust (NFC)

Optimizing away this comparison is not the point of this test,
so make sure it cannot be optimized away.

llvm/test/Transforms/JumpThreading/combine-metadata.ll

@@ -108,7 +108,7 @@ d2:
 d3:
   %y = load i32*, i32** %ptr
   store i32 1, i32* %y
-  %c2 = icmp eq i32* %y, null
+  %c2 = icmp eq i32* %y, @p
   br i1 %c2, label %ret1, label %ret2
 
 ret1:
@@ -118,5 +118,6 @@ ret2:
   ret void
 }
 
+@p = external global i32
 
 !0 = !{}