[Turbofan] Re-enable single splinter.

src/compiler/live-range-separator.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/live-range-separator.h"
 #include "src/compiler/register-allocator.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
@@ skipping 26 matching lines @@
   LifetimePosition end = Min(last_cut, range->End());
 
   if (start < end) {
     // Ensure the original range has a spill range associated, before it gets
     // splintered. Splinters will point to it. This way, when attempting to
     // reuse spill slots of splinters, during allocation, we avoid clobbering
     // such slots.
     if (range->MayRequireSpillRange()) {
       data->CreateSpillRangeForLiveRange(range);
     }
-    TopLevelLiveRange *result = data->NextLiveRange(range->machine_type());
-    DCHECK_NULL(data->live_ranges()[result->vreg()]);
-    data->live_ranges()[result->vreg()] = result;
-
+    if (range->splinter() == nullptr) {
+      TopLevelLiveRange *splinter = data->NextLiveRange(range->machine_type());
+      DCHECK_NULL(data->live_ranges()[splinter->vreg()]);
+      data->live_ranges()[splinter->vreg()] = splinter;
+      range->SetSplinter(splinter);
+    }
     Zone *zone = data->allocation_zone();
-    range->Splinter(start, end, result, zone);
+    range->Splinter(start, end, zone);
   }
 }
 
 
 int FirstInstruction(const UseInterval *interval) {
   LifetimePosition start = interval->start();
   int ret = start.ToInstructionIndex();
   if (start.IsInstructionPosition() && start.IsEnd()) {
     ++ret;
   }
@@ skipping 78 matching lines @@
     int to_remove = range->vreg();
     splinter_parent->Merge(range, data()->allocation_zone());
     data()->live_ranges()[to_remove] = nullptr;
   }
 }
 
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8