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

lib/IR/Verifier.cpp

@@ -1128,7 +1128,7 @@ void Verifier::visitFunction(const Function &F) {
 
     // Check the entry node
     const BasicBlock *Entry = &F.getEntryBlock();
-    Assert1(pred_begin(Entry) == pred_end(Entry),
+    Assert1(pred_empty(Entry),
             "Entry block to function must not have predecessors!", Entry);
 
     // The address of the entry block cannot be taken, unless it is dead.