Support string add in crankshaft:

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
diff --git a/src/ia32/lithium-ia32.cc b/src/ia32/lithium-ia32.cc
index 76a4a063d5e0f163c1e97a7240fc5273d688f082..409e4cd2f7805677ab4c1001c657c1a1454d64b6 100644
--- a/src/ia32/lithium-ia32.cc
+++ b/src/ia32/lithium-ia32.cc
@@ -2001,6 +2001,13 @@ LInstruction* LChunkBuilder::DoStoreNamedGeneric(HStoreNamedGeneric* instr) {
 }
 
 
+LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
+  LOperand* left = UseOrConstantAtStart(instr->left());
+  LOperand* right = UseOrConstantAtStart(instr->right());
+  return MarkAsCall(DefineFixed(new LStringAdd(left, right), eax), instr);
+}
+
+
 LInstruction* LChunkBuilder::DoStringCharCodeAt(HStringCharCodeAt* instr) {
   LOperand* string = UseRegister(instr->string());
   LOperand* index = UseRegisterOrConstant(instr->index());