Unify the handling of comparinsons against null and undefined.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
===================================================================
--- src/arm/lithium-codegen-arm.cc	(revision 9305)
+++ src/arm/lithium-codegen-arm.cc	(working copy)
@@ -1749,25 +1749,35 @@
 }
 
 
-void LCodeGen::DoIsNullAndBranch(LIsNullAndBranch* instr) {
+void LCodeGen::DoIsNilAndBranch(LIsNilAndBranch* instr) {
   Register scratch = scratch0();
   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());
-
-  __ LoadRoot(ip, Heap::kNullValueRootIndex);
+  Heap::RootListIndex nil_value = instr->nil() == kNullValue ?
+      Heap::kNullValueRootIndex :
+      Heap::kUndefinedValueRootIndex;
+  __ LoadRoot(ip, nil_value);
   __ cmp(reg, ip);
-  if (instr->is_strict()) {
+  if (instr->kind() == kStrictEquality) {
     EmitBranch(true_block, false_block, eq);
   } else {
+    Heap::RootListIndex other_nil_value = instr->nil() == kNullValue ?
+        Heap::kUndefinedValueRootIndex :
+        Heap::kNullValueRootIndex;
     Label* true_label = chunk_->GetAssemblyLabel(true_block);
     Label* false_label = chunk_->GetAssemblyLabel(false_block);
     __ b(eq, true_label);
-    __ LoadRoot(ip, Heap::kUndefinedValueRootIndex);
+    __ LoadRoot(ip, other_nil_value);
     __ cmp(reg, ip);
     __ b(eq, true_label);
     __ JumpIfSmi(reg, false_label);