v8: Fix most -Wunreachable-code warnings.

Do this by deleting code after calls to V8_Fatal() (either through
UNREACHABLE() or FATAL()). Comments suggest that the returns there
were needed to make a compiler happy, but all compilers seem to be
happy with this change too. My guess is that either
https://codereview.chromium.org/1393023003 which marked V8_Fatal()
as noreturn, or https://chromium-review.googlesource.com/#/c/544845/
which switched to the C++11 spelling of noreturn, fixed the warnings
that the explicit code after V8_Fatal() was supposed to silence.

There's one more warning in src/compiler/machine-graph-verifier.cc,
but fixing that changes behavior.  I asked about that one in
https://codereview.chromium.org/2573573004/#msg27 instead.

Bug: chromium:346399
Change-Id: Ie9519d5432bdeaaf382e8390d8254d3b79e622e4
Reviewed-on: https://chromium-review.googlesource.com/669803Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48092}

src/api-natives.cc

@@ -689,7 +689,6 @@ Handle<JSFunction> ApiNatives::CreateApiFunction(
       break;
     default:
       UNREACHABLE();
-      type = JS_OBJECT_TYPE;  // Keep the compiler happy.
       break;
   }
 

src/compiler/access-builder.cc

@@ -985,9 +985,6 @@ ElementAccess AccessBuilder::ForTypedArrayElement(ExternalArrayType type,
     }
   }
   UNREACHABLE();
-  ElementAccess access = {kUntaggedBase, 0, Type::None(), MachineType::None(),
-                          kNoWriteBarrier};
-  return access;
 }
 
 // static

src/compiler/bytecode-graph-builder.cc

@@ -2465,7 +2465,6 @@ void BytecodeGraphBuilder::VisitTestTypeOf() {
       break;
     case interpreter::TestTypeOfFlags::LiteralFlag::kOther:
       UNREACHABLE();  // Should never be emitted.
-      result = nullptr;
       break;
   }
   environment()->BindAccumulator(result);

src/deoptimizer.cc

@@ -3318,10 +3318,6 @@ int TranslatedState::CreateNextTranslatedValue(
   }
 
   FATAL("We should never get here - unexpected deopt info.");
-  TranslatedValue translated_value =
-      TranslatedValue(nullptr, TranslatedValue::kInvalid);
-  frame.Add(translated_value);
-  return translated_value.GetChildrenCount();
 }
 
 TranslatedState::TranslatedState(const JavaScriptFrame* frame)

src/dtoa.cc

@@ -68,7 +68,6 @@ void DoubleToAscii(double v, DtoaMode mode, int requested_digits,
       break;
     default:
       UNREACHABLE();
-      fast_worked = false;
   }
   if (fast_worked) return;
 

src/feedback-vector.cc

@@ -288,7 +288,6 @@ Handle<FeedbackVector> FeedbackVector::New(Isolate* isolate,
       case FeedbackSlotKind::kInvalid:
       case FeedbackSlotKind::kKindsNumber:
         UNREACHABLE();
-        vector->set(index, Smi::kZero, SKIP_WRITE_BARRIER);
         break;
     }
     for (int j = 1; j < entry_size; j++) {

src/objects/module.cc

@@ -158,7 +158,6 @@ Cell* Module::GetCell(int cell_index) {
       break;
     case ModuleDescriptor::kInvalid:
       UNREACHABLE();
-      cell = nullptr;
       break;
   }
   return Cell::cast(cell);

src/runtime/runtime-strings.cc

@@ -45,8 +45,6 @@ RUNTIME_FUNCTION(Runtime_GetSubstitution) {
     MaybeHandle<String> GetNamedCapture(Handle<String> name,
                                         CaptureState* state) override {
       UNREACHABLE();
-      *state = INVALID;
-      return MaybeHandle<String>();
     }
 
    private: