[maglev] Implement function context specialization

See also Turbofan's JSContextSpecialization reducer.

For all context loads and stores, this CL implements:

1) depth reduction through graph walks (even without FCS)
2) conversion from the context node to a heap constant
3) if possible, conversion of a load of an immutable context slot load
   to a heap constant

Bug: v8:7700
Change-Id: Ie4d1acd0ff206f25dd5373a860d23b006a31dcee
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3904914Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Linke <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83418}

src/flags/flag-definitions.h

@@ -477,7 +477,7 @@ DEFINE_BOOL(trace_maglev_graph_building, false, "trace maglev graph building")
 DEFINE_BOOL(trace_maglev_regalloc, false, "trace maglev register allocation")
 
 // TODO(v8:7700): Remove once stable.
-DEFINE_BOOL(maglev_function_context_specialization, false,
+DEFINE_BOOL(maglev_function_context_specialization, true,
             "enable function context specialization in maglev")
 
 #if ENABLE_SPARKPLUG

src/maglev/maglev-graph-builder.h

@@ -408,6 +408,13 @@ class MaglevGraphBuilder {
     return node;
   }
 
+  enum ContextSlotMutability { kImmutable, kMutable };
+  bool TrySpecializeLoadContextSlotToFunctionContext(
+      ValueNode** context, size_t* depth, int slot_index,
+      ContextSlotMutability slot_mutability);
+  void BuildLoadContextSlot(ValueNode* context, size_t depth, int slot_index,
+                            ContextSlotMutability slot_mutability);
+
   template <Builtin kBuiltin>
   CallBuiltin* BuildCallBuiltin(std::initializer_list<ValueNode*> inputs) {
     using Descriptor = typename CallInterfaceDescriptorFor<kBuiltin>::type;

src/maglev/maglev-ir.h

@@ -2253,6 +2253,8 @@ class Constant : public FixedInputValueNodeT<0, Constant> {
   void DoLoadToRegister(MaglevAssembler*, OutputRegister);
   Handle<Object> DoReify(LocalIsolate* isolate);
 
+  const compiler::HeapObjectRef& ref() const { return object_; }
+
  private:
   const compiler::HeapObjectRef object_;
 };