[turbofan] enable stack slot reuse

BUG=

Review URL: https://codereview.chromium.org/793683002

Cr-Commit-Position: refs/heads/master@{#25841}

src/compiler/register-allocator.cc

@@ -965,8 +965,8 @@ SpillRange* RegisterAllocator::AssignSpillRangeToLiveRange(LiveRange* range) {
 
 bool RegisterAllocator::TryReuseSpillForPhi(LiveRange* range) {
   DCHECK(FLAG_turbo_reuse_spill_slots);
-  DCHECK(range->HasNoSpillType());
   if (range->IsChild() || !range->is_phi()) return false;
+  DCHECK(range->HasNoSpillType());
 
   auto lookup = phi_map_.find(range->id());
   DCHECK(lookup != phi_map_.end());

src/flag-definitions.h

@@ -402,7 +402,7 @@ DEFINE_IMPLICATION(turbo_inlining_intrinsics, turbo_inlining)
 DEFINE_IMPLICATION(turbo_inlining, turbo_types)
 DEFINE_BOOL(turbo_profiling, false, "enable profiling in TurboFan")
 // TODO(dcarney): this is just for experimentation, remove when default.
-DEFINE_BOOL(turbo_reuse_spill_slots, false, "reuse spill slots in TurboFan")
+DEFINE_BOOL(turbo_reuse_spill_slots, true, "reuse spill slots in TurboFan")
 // TODO(dcarney): this is just for experimentation, remove when default.
 DEFINE_BOOL(turbo_delay_ssa_decon, false,
             "delay ssa deconstruction in TurboFan register allocator")