Fix a stub signature. HeuristicReduce should return a bool.

This should fix a -Wdocumentation warning.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162458 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/PBQP/HeuristicBase.h

@@ -193,8 +193,9 @@ namespace PBQP {
     ///        reduce list.
     /// @return True if a reduction takes place, false if the heuristic reduce
     ///         list is empty.
-    void heuristicReduce() {
+    bool heuristicReduce() {
       llvm_unreachable("Must be implemented in derived class.");
+      return false;
     }
 
     /// \brief Prepare a change in the costs on the given edge.