Introduce a hydrogen value for contexts, support context slot assignment.

src/hydrogen-instructions.cc

Index: src/hydrogen-instructions.cc
diff --git a/src/hydrogen-instructions.cc b/src/hydrogen-instructions.cc
index 2f8b6e263dde9cb8817f7b0f3c3a47619e776cdb..58c7b053d3b99954f668357bc59911befc0c43c1 100644
--- a/src/hydrogen-instructions.cc
+++ b/src/hydrogen-instructions.cc
@@ -1192,7 +1192,15 @@ void HStoreGlobal::PrintDataTo(StringStream* stream) const {
 
 
 void HLoadContextSlot::PrintDataTo(StringStream* stream) const {
-  stream->Add("(%d, %d)", context_chain_length(), slot_index());
+  value()->PrintNameTo(stream);
+  stream->Add("[%d]", slot_index());
+}
+
+
+void HStoreContextSlot::PrintDataTo(StringStream* stream) const {
+  context()->PrintNameTo(stream);
+  stream->Add("[%d] = ", slot_index());
+  value()->PrintNameTo(stream);
 }