Revert "LibJS: Avoid calling generic Instruction::length() during dispatch"

This reverts commit 8f2ee01e6fb41b85f8966f0112034d95ce25f88e.

Speculative revert, as this appears to severely regress performance on
our JS benchmark runner.

Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -386,7 +386,7 @@ FLATTEN_ON_CLANG void Interpreter::run_bytecode(size_t entry_point)
 #define DISPATCH_NEXT(name)                                                                         \
     do {                                                                                            \
         if constexpr (Op::name::IsVariableLength)                                                   \
-            program_counter += static_cast<Op::name const&>(instruction).length();                  \
+            program_counter += instruction.length();                                                \
         else                                                                                        \
             program_counter += sizeof(Op::name);                                                    \
         auto& next_instruction = *reinterpret_cast<Instruction const*>(&bytecode[program_counter]); \

Libraries/LibJS/Bytecode/Op.h

@@ -313,7 +313,6 @@ public:
     {
         return round_up_to_power_of_two(alignof(void*), sizeof(*this) + sizeof(Operand) * m_excluded_names_count);
     }
-    size_t length() const { return length_impl(); }
 
     Operand dst() const { return m_dst; }
     Operand from_object() const { return m_from_object; }
@@ -363,7 +362,6 @@ public:
     {
         return round_up_to_power_of_two(alignof(void*), sizeof(*this) + sizeof(Operand) * m_element_count);
     }
-    size_t length() const { return length_impl(); }
 
     size_t element_count() const { return m_element_count; }
 
@@ -390,7 +388,6 @@ public:
     {
         return round_up_to_power_of_two(alignof(void*), sizeof(*this) + sizeof(Value) * m_element_count);
     }
-    size_t length() const { return length_impl(); }
 
     void execute_impl(Bytecode::Interpreter&) const;
     ByteString to_byte_string_impl(Bytecode::Executable const&) const;
@@ -1819,7 +1816,6 @@ public:
     {
         return round_up_to_power_of_two(alignof(void*), sizeof(*this) + sizeof(Operand) * m_argument_count);
     }
-    size_t length() const { return length_impl(); }
 
     Operand dst() const { return m_dst; }
     Operand callee() const { return m_callee; }
@@ -1869,7 +1865,6 @@ public:
     {
         return round_up_to_power_of_two(alignof(void*), sizeof(*this) + sizeof(Operand) * m_argument_count);
     }
-    size_t length() const { return length_impl(); }
 
     Operand dst() const { return m_dst; }
     Operand callee() const { return m_callee; }
@@ -1921,7 +1916,6 @@ public:
     {
         return round_up_to_power_of_two(alignof(void*), sizeof(*this) + sizeof(Operand) * m_argument_count);
     }
-    size_t length() const { return length_impl(); }
 
     Operand dst() const { return m_dst; }
     Operand callee() const { return m_callee; }
@@ -1970,7 +1964,6 @@ public:
     {
         return round_up_to_power_of_two(alignof(void*), sizeof(*this) + sizeof(Operand) * m_argument_count);
     }
-    size_t length() const { return length_impl(); }
 
     Operand dst() const { return m_dst; }
     Operand callee() const { return m_callee; }
@@ -2088,7 +2081,6 @@ public:
     {
         return round_up_to_power_of_two(alignof(void*), sizeof(*this) + sizeof(Optional<Operand>) * m_element_keys_count);
     }
-    size_t length() const { return length_impl(); }
 
     ThrowCompletionOr<void> execute_impl(Bytecode::Interpreter&) const;
     ByteString to_byte_string_impl(Bytecode::Executable const&) const;