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/BBVectorize.cpp

@@ -1277,7 +1277,7 @@ namespace {
             CostSavings, FixedOrder)) continue;
 
         // J is a candidate for merging with I.
-        if (!PairableInsts.size() ||
+        if (PairableInsts.empty() ||
              PairableInsts[PairableInsts.size()-1] != I) {
           PairableInsts.push_back(I);
         }