YJIT: Fix crash when Kernel is prepended before JIT init

Use RCLASS_ORIGIN when looking up methods to register for specialized
codegen. Module#prepend moves original methods to an origin iclass,
so rb_method_entry_at on the module itself returns NULL after prepend.

This happens in practice when BUNDLER_SETUP triggers Kernel.prepend
during ruby_init_prelude, before rb_yjit_init runs.

Author: rwstauner

yjit/src/codegen.rs

@@ -10910,7 +10910,8 @@ pub fn yjit_reg_method_codegen_fns() {
 /// See also: [lookup_cfunc_codegen].
 fn reg_method_codegen(klass: VALUE, method_name: &str, gen_fn: MethodGenFn) {
     let mid = unsafe { rb_intern2(method_name.as_ptr().cast(), method_name.len().try_into().unwrap()) };
-    let me = unsafe { rb_method_entry_at(klass, mid) };
+    // Use RCLASS_ORIGIN to find methods that were moved by Module#prepend
+    let me = unsafe { rb_method_entry_at(RCLASS_ORIGIN(klass), mid) };
 
     if me.is_null() {
         panic!("undefined optimized method!: {method_name}");