[ic] Temporary resurrect ICStateField to recover performance regression.

Zero value of ICStateField part made megamorphic stub cache unusable.
We need to revisit hashing algorithm in order to remove the ICStateField.

BUG=chromium:619016
LOG=N

Review-Url: https://codereview.chromium.org/2064713002
Cr-Commit-Position: refs/heads/master@{#36959}

src/objects-inl.h

@@ -5119,8 +5119,10 @@ Address Code::constant_pool() {
 
 Code::Flags Code::ComputeFlags(Kind kind, ExtraICState extra_ic_state,
                                CacheHolderFlag holder) {
+  // TODO(ishell): remove ICStateField.
   // Compute the bit mask.
   unsigned int bits = KindField::encode(kind) |
+                      ICStateField::encode(MONOMORPHIC) |
                       ExtraICStateField::encode(extra_ic_state) |
                       CacheHolderField::encode(holder);
   return static_cast<Flags>(bits);

src/objects.h

@@ -5281,7 +5281,9 @@ class Code: public HeapObject {
   class ProfilerTicksField : public BitField<int, 4, 28> {};
 
   // Flags layout.  BitField<type, shift, size>.
-  class CacheHolderField : public BitField<CacheHolderFlag, 0, 2> {};
+  class ICStateField : public BitField<InlineCacheState, 0, 3> {};
+  class CacheHolderField
+      : public BitField<CacheHolderFlag, ICStateField::kNext, 2> {};
   class KindField : public BitField<Kind, CacheHolderField::kNext, 5> {};
   class ExtraICStateField : public BitField<ExtraICState, KindField::kNext,
                                             PlatformSmiTagging::kSmiValueSize -