Perform string add in generated code on X64 platform...

src/ia32/macro-assembler-ia32.cc

Index: src/ia32/macro-assembler-ia32.cc
===================================================================
--- src/ia32/macro-assembler-ia32.cc	(revision 3430)
+++ src/ia32/macro-assembler-ia32.cc	(working copy)
@@ -834,10 +834,9 @@
   // Calculate the number of bytes needed for the characters in the string while
   // observing object alignment.
   ASSERT((SeqTwoByteString::kHeaderSize & kObjectAlignmentMask) == 0);
-  mov(scratch1, length);
   ASSERT(kShortSize == 2);
-  shl(scratch1, 1);
-  add(Operand(scratch1), Immediate(kObjectAlignmentMask));
+  // scratch1 = length * 2 + kObjectAlignmentMask.
+  lea(scratch1, Operand(length, length, times_1, kObjectAlignmentMask));
   and_(Operand(scratch1), Immediate(~kObjectAlignmentMask));
 
   // Allocate two byte string in new space.