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!

llvm-svn: 226161
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 72e0aca..a1f425e 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -2340,7 +2340,7 @@
   }
 
   // If we have multiple alternative constraints, select the best alternative.
-  if (ConstraintOperands.size()) {
+  if (!ConstraintOperands.empty()) {
     if (maCount) {
       unsigned bestMAIndex = 0;
       int bestWeight = -1;