ScopDetection: Simplify std::distance(....) to BB->size()

Suggested by: Johannes Doerfert <doerfert@cs.uni-saarland.de>

llvm-svn: 256260
diff --git a/polly/lib/Analysis/ScopDetection.cpp b/polly/lib/Analysis/ScopDetection.cpp
index 1083507..7e8075d 100644
--- a/polly/lib/Analysis/ScopDetection.cpp
+++ b/polly/lib/Analysis/ScopDetection.cpp
@@ -1150,7 +1150,7 @@
 
   for (auto *BB : Context.CurRegion.blocks())
     if (Context.CurRegion.contains(LI->getLoopFor(BB)))
-      InstCount += std::distance(BB->begin(), BB->end());
+      InstCount += BB->size();
 
   InstCount = InstCount / NumLoops;