[maglev] Put stack slot reuse behind a flag

It is currently incorrect and causing issues, put it behind a flag so
that we can fix these issues while working on the rest of maglev in
parallel.

Bug: v8:7700
Change-Id: Idab7056db1236366410c30c06473016842aee5ab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3748659
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81612}

src/flags/flag-definitions.h

@@ -509,6 +509,8 @@ DEFINE_BOOL(future, FUTURE_BOOL,
 DEFINE_BOOL(maglev, false, "enable the maglev optimizing compiler")
 DEFINE_BOOL(maglev_inlining, false,
             "enable inlining in the maglev optimizing compiler")
+DEFINE_BOOL(maglev_reuse_stack_slots, false,
+            "reuse stack slots in the maglev optimizing compiler")
 #else
 #define V8_ENABLE_MAGLEV_BOOL false
 DEFINE_BOOL_READONLY(maglev, false, "enable the maglev optimizing compiler")

src/maglev/maglev-regalloc.cc

@@ -1085,7 +1085,7 @@ void StraightForwardRegisterAllocator::AllocateSpillSlot(ValueNode* node) {
   // architectures.
   SpillSlots& slots = is_tagged ? tagged_ : untagged_;
   MachineRepresentation representation = node->GetMachineRepresentation();
-  if (slots.free_slots.empty()) {
+  if (!FLAG_maglev_reuse_stack_slots || slots.free_slots.empty()) {
     free_slot = slots.top++;
   } else {
     NodeIdT start = node->live_range().start;