YJIT: Fix BorrowMutError on BOP invalidation

[k0kubun]

This fixes:

thread '<unnamed>' panicked at 'already borrowed: BorrowMutError', src/core.rs:509:16
stack backtrace:
   0: rust_begin_unwind
             at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/panicking.rs:498:5
   1: core::panicking::panic_fmt
             at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/core/src/panicking.rs:107:14
   2: core::result::unwrap_failed
             at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/core/src/result.rs:1613:5
   3: core::result::Result<T,E>::expect
             at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/core/src/result.rs:1255:23
   4: core::cell::RefCell<T>::borrow_mut
             at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/core/src/cell.rs:946:9
   5: yjit::core::BlockRef::borrow_mut
             at ./yjit/src/core.rs:509:9
   6: yjit::core::regenerate_branch
             at ./yjit/src/core.rs:1708:21
   7: yjit::core::invalidate_block_version
             at ./yjit/src/core.rs:2370:9
   8: yjit::invariants::rb_yjit_bop_redefined::{{closure}}
             at ./yjit/src/invariants.rs:229:17
   9: std::panicking::try::do_call
             at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/panicking.rs:406:40
  10: __rust_try
  11: std::panicking::try
             at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/panicking.rs:370:19
  12: std::panic::catch_unwind
             at /rustc/02072b482a8b5357f7fb5e5637444ae30e423c40/library/std/src/panic.rs:133:14
  13: yjit::cruby::with_vm_lock
             at ./yjit/src/cruby.rs:617:21
  14: rb_yjit_bop_redefined
             at ./yjit/src/invariants.rs:221:5

https://cirrus-ci.com/task/6461636395925504?logs=make_test_all