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/Transforms/Scalar/Reg2Mem.cpp

@@ -73,7 +73,7 @@ bool RegToMem::runOnFunction(Function &F) {
 
   // Insert all new allocas into entry block.
   BasicBlock *BBEntry = &F.getEntryBlock();
-  assert(pred_begin(BBEntry) == pred_end(BBEntry) &&
+  assert(pred_empty(BBEntry) &&
          "Entry block to function must not have predecessors!");
 
   // Find first non-alloca instruction and create insertion point. This is