LibJS: Add caching of this value in ResolveThisBinding instruction

Because "this" value cannot be changed during function execution it is
safe to compute it once and then use for future access.

This optimization makes ai-astar.js run 8% faster.

Author: kalenikaliaksandr

Userland/Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -208,6 +208,8 @@ Interpreter::ValueAndFrame Interpreter::run_and_return_frame(Realm& realm, Execu
     else
         push_call_frame(make<CallFrame>(), executable.number_of_registers);
 
+    TemporaryChange restore_this_value { m_this_value, {} };
+
     for (;;) {
         Bytecode::InstructionStreamIterator pc(m_current_block->instruction_stream());
         TemporaryChange temp_change { m_pc, &pc };

Userland/Libraries/LibJS/Bytecode/Interpreter.h

@@ -95,6 +95,8 @@ class Interpreter {
 
     VM::InterpreterExecutionScope ast_interpreter_scope(Realm&);
 
+    Optional<Value>& this_value() { return m_this_value; }
+
     void visit_edges(Cell::Visitor&);
 
 private:
@@ -119,6 +121,7 @@ class Interpreter {
     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;
     Optional<Value> m_saved_exception;
     Executable* m_current_executable { nullptr };

Userland/Libraries/LibJS/Bytecode/Op.cpp

@@ -732,8 +732,13 @@ ThrowCompletionOr<void> Jump::execute_impl(Bytecode::Interpreter& interpreter) c
 
 ThrowCompletionOr<void> ResolveThisBinding::execute_impl(Bytecode::Interpreter& interpreter) const
 {
-    auto& vm = interpreter.vm();
-    interpreter.accumulator() = TRY(vm.resolve_this_binding());
+    if (!interpreter.this_value().has_value()) {
+        // 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());
+    }
+    interpreter.accumulator() = interpreter.this_value().value();
     return {};
 }