LibJS: Keep cached this value in a call frame register

Just moving more things to call frame registers..

Userland/Libraries/LibJS/Bytecode/Generator.h

@@ -252,7 +252,7 @@ private:
     NonnullOwnPtr<IdentifierTable> m_identifier_table;
     NonnullOwnPtr<RegexTable> m_regex_table;
 
-    u32 m_next_register { 3 };
+    u32 m_next_register { Register::reserved_register_count };
     u32 m_next_block { 1 };
     u32 m_next_property_lookup_cache { 0 };
     u32 m_next_global_variable_cache { 0 };

Userland/Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -179,8 +179,6 @@ Interpreter::ValueAndFrame Interpreter::run_and_return_frame(Executable& executa
     else
         push_call_frame(make<CallFrame>(), executable.number_of_registers);
 
-    TemporaryChange restore_this_value { m_this_value, {} };
-
     for (;;) {
         auto pc = InstructionStreamIterator { m_current_block->instruction_stream(), m_current_executable };
         TemporaryChange temp_change { m_pc, Optional<InstructionStreamIterator&>(pc) };

Userland/Libraries/LibJS/Bytecode/Interpreter.h

@@ -90,8 +90,6 @@ public:
     auto& instruction_stream_iterator() const { return m_pc; }
     DeprecatedString debug_position() const;
 
-    Optional<Value>& this_value() { return m_this_value; }
-
     void visit_edges(Cell::Visitor&);
 
 private:
@@ -116,7 +114,6 @@ private:
     Span<Value> m_current_call_frame;
     Optional<BasicBlock const*> m_pending_jump;
     BasicBlock const* m_scheduled_jump { nullptr };
-    Optional<Value> m_this_value;
     Optional<Value> m_return_value;
     Executable* m_current_executable { nullptr };
     BasicBlock const* m_current_block { nullptr };

Userland/Libraries/LibJS/Bytecode/Op.cpp

@@ -767,13 +767,14 @@ ThrowCompletionOr<void> Jump::execute_impl(Bytecode::Interpreter& interpreter) c
 
 ThrowCompletionOr<void> ResolveThisBinding::execute_impl(Bytecode::Interpreter& interpreter) const
 {
-    if (!interpreter.this_value().has_value()) {
+    auto& cached_this_value = interpreter.reg(Register::this_value());
+    if (cached_this_value.is_empty()) {
         // OPTIMIZATION: Because the value of 'this' cannot be reassigned during a function execution, it's
         //               resolved once and then saved for subsequent use.
         auto& vm = interpreter.vm();
-        interpreter.this_value() = TRY(vm.resolve_this_binding());
+        cached_this_value = TRY(vm.resolve_this_binding());
     }
-    interpreter.accumulator() = interpreter.this_value().value();
+    interpreter.accumulator() = cached_this_value;
     return {};
 }
 

Userland/Libraries/LibJS/Bytecode/Register.h

@@ -33,6 +33,14 @@ public:
         return Register(exception_index);
     }
 
+    static constexpr Register this_value()
+    {
+        constexpr u32 this_value_index = 3;
+        return Register(this_value_index);
+    }
+
+    static constexpr u32 reserved_register_count = 4;
+
     constexpr explicit Register(u32 index)
         : m_index(index)
     {