Change the x64 optimizing compiler to stop using the fixed context register rsi.

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
diff --git a/src/ia32/lithium-codegen-ia32.cc b/src/ia32/lithium-codegen-ia32.cc
index 31e9dd82e2d432bbd6fc2a512f49cfe73b72359c..deb976baed6bd1f72deaed891a6d5c5494e7373f 100644
--- a/src/ia32/lithium-codegen-ia32.cc
+++ b/src/ia32/lithium-codegen-ia32.cc
@@ -203,11 +203,8 @@ bool LCodeGen::GeneratePrologue() {
         // Store it in the context.
         int context_offset = Context::SlotOffset(var->index());
         __ mov(Operand(esi, context_offset), eax);
-        // Update the write barrier. This clobbers all involved
-        // registers, so we have to use a third register to avoid
-        // clobbering esi.
-        __ mov(ecx, esi);
-        __ RecordWrite(ecx, context_offset, eax, ebx);
+        // Update the write barrier.
+        __ RecordWrite(esi, context_offset, eax, ebx);
       }
     }
     Comment(";;; End allocate local context");