Replace size method call of containers to empty method where appropriate

This patch was generated by a clang tidy checker that is being open sourced.
The documentation of that checker is the following:

/// The emptiness of a container should be checked using the empty method
/// instead of the size method. It is not guaranteed that size is a
/// constant-time function, and it is generally more efficient and also shows
/// clearer intent to use empty. Furthermore some containers may implement the
/// empty method but not implement the size method. Using empty whenever
/// possible makes it easier to switch to another container in the future.

Patch by Gábor Horváth!



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226161 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -1760,7 +1760,7 @@ int BoUpSLP::getTreeCost() {
 
   // We only vectorize tiny trees if it is fully vectorizable.
   if (VectorizableTree.size() < 3 && !isFullyVectorizableTinyTree()) {
-    if (!VectorizableTree.size()) {
+    if (VectorizableTree.empty()) {
       assert(!ExternalUses.size() && "We should not have any external users");
     }
     return INT_MAX;