Remove remaining uses of null's absolute address from non-IA32.

runtime/vm/intrinsifier_mips.cc

Index: runtime/vm/intrinsifier_mips.cc
diff --git a/runtime/vm/intrinsifier_mips.cc b/runtime/vm/intrinsifier_mips.cc
index d5df53304f102d24ec04997f901c3346bd6c0eb2..95783b6407e07544dde12161cf5c42698fb2c834 100644
--- a/runtime/vm/intrinsifier_mips.cc
+++ b/runtime/vm/intrinsifier_mips.cc
@@ -58,7 +58,7 @@ void Intrinsifier::ObjectArraySetIndexed(Assembler* assembler) {
     __ lw(T2, Address(SP, 0 * kWordSize));  // Value.
 
     // Null value is valid for any type.
-    __ LoadImmediate(T7, reinterpret_cast<int32_t>(Object::null()));
+    __ LoadObject(T7, Object::null_object());
     __ beq(T2, T7, &checked_ok);
 
     __ lw(T1, Address(SP, 2 * kWordSize));  // Array.
@@ -169,7 +169,7 @@ void Intrinsifier::GrowableArray_add(Assembler* assembler) {
   __ StoreIntoObject(T2,
                      FieldAddress(T1, Array::data_offset()),
                      T0);
-  __ LoadImmediate(T7, reinterpret_cast<int32_t>(Object::null()));
+  __ LoadObject(T7, Object::null_object());
   __ Ret();
   __ delay_slot()->mov(V0, T7);
   __ Bind(&fall_through);