Standardize {pred,succ,use,user}_empty()

The functions {pred,succ,use,user}_{begin,end} exist, but many users
have to check *_begin() with *_end() by hand to determine if the
BasicBlock or User is empty. Fix this with a standard *_empty(),
demonstrating a few usecases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225760 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Value.h

@@ -286,6 +286,8 @@ public:
     return iterator_range<const_use_iterator>(use_begin(), use_end());
   }
 
+  bool               user_empty() const { return UseList == nullptr; }
+
   typedef user_iterator_impl<User>       user_iterator;
   typedef user_iterator_impl<const User> const_user_iterator;
   user_iterator       user_begin()       { return user_iterator(UseList); }