Set kChangesNewSpacePromotion for HStringAdd

TEST=none
BUG=none

Review URL: https://codereview.chromium.org/11143006
Patch from Derek J Conrod <dconrod@codeaurora.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12822 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

AUTHORS

@@ -20,6 +20,7 @@ Burcu Dogan <burcujdogan@gmail.com>
 Craig Schlenter <craig.schlenter@gmail.com>
 Daniel Andersson <kodandersson@gmail.com>
 Daniel James <dnljms@gmail.com>
+Derek J Conrod <dconrod@codeaurora.org>
 Dineel D Sule <dsule@codeaurora.org>
 Erich Ocean <erich.ocean@me.com>
 Fedor Indutny <fedor@indutny.com>

src/hydrogen-instructions.h

@@ -4805,6 +4805,7 @@ class HStringAdd: public HBinaryOperation {
     set_representation(Representation::Tagged());
     SetFlag(kUseGVN);
     SetGVNFlag(kDependsOnMaps);
+    SetGVNFlag(kChangesNewSpacePromotion);
   }
 
   virtual Representation RequiredInputRepresentation(int index) {