[turbofan] First step towards sanitizing for-in and making it optimizable.

src/mips/full-codegen-mips.cc

Index: src/mips/full-codegen-mips.cc
diff --git a/src/mips/full-codegen-mips.cc b/src/mips/full-codegen-mips.cc
index 0c00f713402583b4f5940020901e50d79900419d..c81f346d13cc86a87c4fcb95d105ce9f673f68b0 100644
--- a/src/mips/full-codegen-mips.cc
+++ b/src/mips/full-codegen-mips.cc
@@ -1259,10 +1259,11 @@ void FullCodeGenerator::VisitForInStatement(ForInStatement* stmt) {
   // any more. If the property has been removed while iterating, we
   // just skip it.
   __ Push(a1, a3);  // Enumerable and current entry.
-  __ InvokeBuiltin(Builtins::FILTER_KEY, CALL_FUNCTION);
+  __ CallRuntime(Runtime::kForInFilter, 2);
   PrepareForBailoutForId(stmt->FilterId(), TOS_REG);
   __ mov(a3, result_register());
-  __ Branch(loop_statement.continue_label(), eq, a3, Operand(zero_reg));
+  __ LoadRoot(at, Heap::kUndefinedValueRootIndex);
+  __ Branch(loop_statement.continue_label(), eq, a3, Operand(at));
 
   // Update the 'each' property or variable from the possibly filtered
   // entry in register a3.