LibJS/Bytecode: Remove Instruction::m_length field

Now that the interpreter is unrolled, we can advance the program counter
manually based on the current instruction type.

This makes most instructions a bit smaller. :^)

Userland/Libraries/LibJS/Bytecode/Instruction.cpp

@@ -10,13 +10,6 @@
 
 namespace JS::Bytecode {
 
-Instruction::Instruction(Type type, size_t length)
-    : m_type(type)
-    , m_length(length)
-{
-    VERIFY(length <= NumericLimits<u32>::max());
-}
-
 void Instruction::destroy(Instruction& instruction)
 {
 #define __BYTECODE_OP(op)                        \
@@ -49,6 +42,42 @@ void Instruction::visit_labels(Function<void(JS::Bytecode::Label&)> visitor)
 #undef __BYTECODE_OP
 }
 
+template<typename Op>
+concept HasVariableLength = Op::IsVariableLength;
+
+template<typename Op>
+concept HasFixedLength = !Op::IsVariableLength;
+
+template<HasVariableLength Op>
+size_t get_length_impl(Op const& op)
+{
+    return op.length_impl();
+}
+
+// Function template for types without a length_impl method
+template<HasFixedLength Op>
+size_t get_length_impl(Op const&)
+{
+    return sizeof(Op);
+}
+
+size_t Instruction::length() const
+{
+#define __BYTECODE_OP(op)                                   \
+    case Type::op: {                                        \
+        auto& typed_op = static_cast<Op::op const&>(*this); \
+        return get_length_impl(typed_op);                   \
+    }
+
+    switch (type()) {
+        ENUMERATE_BYTECODE_OPS(__BYTECODE_OP)
+    default:
+        VERIFY_NOT_REACHED();
+    }
+
+#undef __BYTECODE_OP
+}
+
 UnrealizedSourceRange InstructionStreamIterator::source_range() const
 {
     VERIFY(m_executable);