Unify the handling of comparinsons against null and undefined.

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
===================================================================
--- src/ia32/lithium-codegen-ia32.cc	(revision 9305)
+++ src/ia32/lithium-codegen-ia32.cc	(working copy)
@@ -1583,23 +1583,33 @@
 }
 
 
-void LCodeGen::DoIsNullAndBranch(LIsNullAndBranch* instr) {
+void LCodeGen::DoIsNilAndBranch(LIsNilAndBranch* instr) {
   Register reg = ToRegister(instr->InputAt(0));
+  int false_block = chunk_->LookupDestination(instr->false_block_id());
 
-  // TODO(fsc): If the expression is known to be a smi, then it's
-  // definitely not null. Jump to the false block.
+  // If the expression is known to be untagged or a smi, then it's definitely
+  // not null, and it can't be a an undetectable object.
+  if (instr->hydrogen()->representation().IsSpecialization() ||
+      instr->hydrogen()->type().IsSmi()) {
+    EmitGoto(false_block);
+    return;
+  }
 
   int true_block = chunk_->LookupDestination(instr->true_block_id());
-  int false_block = chunk_->LookupDestination(instr->false_block_id());
-
-  __ cmp(reg, factory()->null_value());
-  if (instr->is_strict()) {
+  Handle<Object> nil_value = instr->nil() == kNullValue ?
+      factory()->null_value() :
+      factory()->undefined_value();
+  __ cmp(reg, nil_value);
+  if (instr->kind() == kStrictEquality) {
     EmitBranch(true_block, false_block, equal);
   } else {
+    Handle<Object> other_nil_value = instr->nil() == kNullValue ?
+        factory()->undefined_value() :
+        factory()->null_value();
     Label* true_label = chunk_->GetAssemblyLabel(true_block);
     Label* false_label = chunk_->GetAssemblyLabel(false_block);
     __ j(equal, true_label);
-    __ cmp(reg, factory()->undefined_value());
+    __ cmp(reg, other_nil_value);
     __ j(equal, true_label);
     __ JumpIfSmi(reg, false_label);
     // Check for undetectable objects by looking in the bit field in