[Liftoff] [cleanup] Use default copy and move semantics

Just use the default operators instead of reimplementing them for
{Steal} and {Split}.

Drive-by: Remove unactionable TODO.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I7556cbf7264cf271b2e8966a5e96ca8e41eb3e73
Reviewed-on: https://chromium-review.googlesource.com/789862
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49640}

src/wasm/baseline/liftoff-assembler.cc

@@ -242,19 +242,13 @@ void LiftoffAssembler::CacheState::InitMerge(const CacheState& source,
 }
 
 void LiftoffAssembler::CacheState::Steal(CacheState& source) {
-  stack_state.swap(source.stack_state);
-  used_registers = source.used_registers;
-  memcpy(register_use_count, source.register_use_count,
-         sizeof(register_use_count));
-  last_spilled_reg = source.last_spilled_reg;
+  // Just use the move assignment operator.
+  *this = std::move(source);
 }
 
 void LiftoffAssembler::CacheState::Split(const CacheState& source) {
-  stack_state = source.stack_state;
-  used_registers = source.used_registers;
-  memcpy(register_use_count, source.register_use_count,
-         sizeof(register_use_count));
-  last_spilled_reg = source.last_spilled_reg;
+  // Call the private copy assignment operator.
+  *this = source;
 }
 
 LiftoffAssembler::LiftoffAssembler(Isolate* isolate)

src/wasm/baseline/liftoff-assembler.h

@@ -127,9 +127,11 @@ class LiftoffAssembler : public TurboAssembler {
   static_assert(IS_TRIVIALLY_COPYABLE(VarState),
                 "VarState should be trivially copyable");
 
-  // TODO(clemensh): Make this a proper class.
   struct CacheState {
-    MOVE_ONLY_WITH_DEFAULT_CONSTRUCTORS(CacheState);
+    // Allow default construction, move construction, and move assignment.
+    CacheState() = default;
+    CacheState(CacheState&&) = default;
+    CacheState& operator=(CacheState&&) = default;
 
     // TODO(clemensh): Improve memory management here; avoid std::vector.
     std::vector<VarState> stack_state;
@@ -208,6 +210,12 @@ class LiftoffAssembler : public TurboAssembler {
           Register::from_code(base::bits::CountTrailingZeros(remaining_regs));
       return last_spilled_reg;
     }
+
+   private:
+    // Make the copy assignment operator private (to be used from {Split()}).
+    CacheState& operator=(const CacheState&) = default;
+    // Disallow copy construction.
+    CacheState(const CacheState&) = delete;
   };
 
   Register PopToRegister(ValueType, PinnedRegisterScope = {});