MIPS64: Reland r22082 "Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/mips64/stub-cache-mips64.cc

Index: src/mips64/stub-cache-mips64.cc
diff --git a/src/mips64/stub-cache-mips64.cc b/src/mips64/stub-cache-mips64.cc
index 5260eb3e95a724aaa0a85499da2e5bfd75a2f348..8a6b02eece79e02c037c1cea77a858acfc4848d7 100644
--- a/src/mips64/stub-cache-mips64.cc
+++ b/src/mips64/stub-cache-mips64.cc
@@ -414,8 +414,9 @@ void StoreStubCompiler::GenerateStoreTransition(MacroAssembler* masm,
     }
   } else if (representation.IsDouble()) {
     Label do_store, heap_number;
-    __ LoadRoot(scratch3, Heap::kHeapNumberMapRootIndex);
-    __ AllocateHeapNumber(storage_reg, scratch1, scratch2, scratch3, slow);
+    __ LoadRoot(scratch3, Heap::kMutableHeapNumberMapRootIndex);
+    __ AllocateHeapNumber(storage_reg, scratch1, scratch2, scratch3, slow,
+                          TAG_RESULT, MUTABLE);
 
     __ JumpIfNotSmi(value_reg, &heap_number);
     __ SmiUntag(scratch1, value_reg);