VM: Fix build on GCC which requires explicit initialization of the base class

in the copy constructor.

TBR=fschneider@google.com

Review-Url: https://codereview.chromium.org/2738733002 .

runtime/vm/intermediate_language.h

@@ -56,7 +56,8 @@ class CompileType : public ZoneAllocated {
       : is_nullable_(is_nullable), cid_(cid), type_(type) {}
 
   CompileType(const CompileType& other)
-      : is_nullable_(other.is_nullable_),
+      : ZoneAllocated(),
+        is_nullable_(other.is_nullable_),
         cid_(other.cid_),
         type_(other.type_) {}