[turbofan] removed some dead code.

In a previous incarnation of live range merging, we needed to cache the
last child in a chain of live ranges. We don't anymore, so removing
unused code.

BUG=

Review URL: https://codereview.chromium.org/1536523003

Cr-Commit-Position: refs/heads/master@{#32931}

src/compiler/register-allocator.cc

@@ -441,9 +441,6 @@ LiveRange* LiveRange::SplitAt(LifetimePosition position, Zone* zone) {
   child->top_level_ = TopLevel();
   child->next_ = next_;
   next_ = child;
-  if (child->next() == nullptr) {
-    TopLevel()->set_last_child(child);
-  }
   return child;
 }
 
@@ -540,15 +537,6 @@ UsePosition* LiveRange::DetachAt(LifetimePosition position, LiveRange* result,
 }
 
 
-void LiveRange::AppendAsChild(TopLevelLiveRange* other) {
-  next_ = other;
-
-  other->UpdateParentForAllChildren(TopLevel());
-  TopLevel()->UpdateSpillRangePostMerge(other);
-  TopLevel()->set_last_child(other->last_child());
-}
-
-
 void LiveRange::UpdateParentForAllChildren(TopLevelLiveRange* new_top_level) {
   LiveRange* child = this;
   for (; child != nullptr; child = child->next()) {
@@ -709,7 +697,6 @@ TopLevelLiveRange::TopLevelLiveRange(int vreg, MachineRepresentation rep)
       spill_move_insertion_locations_(nullptr),
       spilled_in_deferred_blocks_(false),
       spill_start_index_(kMaxInt),
-      last_child_(this),
       last_pos_(nullptr),
       splinter_(nullptr),
       has_preassigned_slot_(false) {

src/compiler/register-allocator.h

@@ -423,7 +423,6 @@ class LiveRange : public ZoneObject {
   explicit LiveRange(int relative_id, MachineRepresentation rep,
                      TopLevelLiveRange* top_level);
 
-  void AppendAsChild(TopLevelLiveRange* other);
   void UpdateParentForAllChildren(TopLevelLiveRange* new_top_level);
 
   void set_spilled(bool value) { bits_ = SpilledField::update(bits_, value); }
@@ -610,8 +609,6 @@ class TopLevelLiveRange final : public LiveRange {
   SpillMoveInsertionList* spill_move_insertion_locations() const {
     return spill_move_insertion_locations_;
   }
-  void set_last_child(LiveRange* range) { last_child_ = range; }
-  LiveRange* last_child() const { return last_child_; }
   TopLevelLiveRange* splinter() const { return splinter_; }
   void SetSplinter(TopLevelLiveRange* splinter) {
     DCHECK_NULL(splinter_);
@@ -647,7 +644,6 @@ class TopLevelLiveRange final : public LiveRange {
   // just for spill in a single deferred block.
   bool spilled_in_deferred_blocks_;
   int spill_start_index_;
-  LiveRange* last_child_;
   UsePosition* last_pos_;
   TopLevelLiveRange* splinter_;
   bool has_preassigned_slot_;