MIPS64: Megamorphic KeyedLoadIC needs special handling for vector ics.

Port 16843e23

Original commit message:
When --vector-ics is true, we still tail-call to the hand-written
megamorphic KeyedLoadIC (formerly "generic"). Now that this code uses
the megamorphic cache, it needs to deal properly with the vector and
slot registers. Achieve this with a sentinel vectors/slot combo.

BUG=

Review URL: https://codereview.chromium.org/894053002

Cr-Commit-Position: refs/heads/master@{#26384}

src/ic/mips64/ic-mips64.cc

@@ -534,10 +534,24 @@ void KeyedLoadIC::GenerateMegamorphic(MacroAssembler* masm) {
   __ LoadRoot(at, Heap::kHashTableMapRootIndex);
   __ Branch(&probe_dictionary, eq, a4, Operand(at));
 
+  if (FLAG_vector_ics) {
+    // When vector ics are in use, the handlers in the stub cache expect a
+    // vector and slot. Since we won't change the IC from any downstream
+    // misses, a dummy vector can be used.
+    Register vector = VectorLoadICDescriptor::VectorRegister();
+    Register slot = VectorLoadICDescriptor::SlotRegister();
+    DCHECK(!AreAliased(vector, slot, a4, a5, a6, t1));
+    Handle<TypeFeedbackVector> dummy_vector = Handle<TypeFeedbackVector>::cast(
+        masm->isolate()->factory()->keyed_load_dummy_vector());
+    int int_slot = dummy_vector->GetIndex(FeedbackVectorICSlot(0));
+    __ LoadRoot(vector, Heap::kKeyedLoadDummyVectorRootIndex);
+    __ li(slot, Operand(Smi::FromInt(int_slot)));
+  }
+
   Code::Flags flags = Code::RemoveTypeAndHolderFromFlags(
       Code::ComputeHandlerFlags(Code::LOAD_IC));
   masm->isolate()->stub_cache()->GenerateProbe(
-      masm, Code::LOAD_IC, flags, false, receiver, key, a3, a4, a5, a6);
+      masm, Code::LOAD_IC, flags, false, receiver, key, a4, a5, a6, t1);
   // Cache miss.
   GenerateMiss(masm);