YJIT: Fix BorrowMutError on GC.compact

[k0kubun]

Follows up #7151 and #7164.

#7164 downgraded mut borrow of a block to (non-mut) borrow for target.get_blockid(), but target.set_iseq() needed to mut borrow it. So a block shouldn't be borrowed at all when calling target.set_iseq.

We could achieve this by cloning a vector of rc and then dropping the block. This addresses the following error:

thread '<unnamed>' panicked at 'already borrowed: BorrowMutError', src/core.rs:509:16
  stack backtrace:
     0: rust_begin_unwind
               at /rustc/90743e7298aca107ddaa0c202a4d3604e29bfeb6/library/std/src/panicking.rs:575:5
     1: core::panicking::panic_fmt
               at /rustc/90743e7298aca107ddaa0c202a4d3604e29bfeb6/library/core/src/panicking.rs:65:14
     2: core::result::unwrap_failed
               at /rustc/90743e7298aca107ddaa0c202a4d3604e29bfeb6/library/core/src/result.rs:1791:5
     3: core::result::Result<T,E>::expect
               at /rustc/90743e7298aca107ddaa0c202a4d3604e29bfeb6/library/core/src/result.rs:1070:23
     4: core::cell::RefCell<T>::borrow_mut
               at /rustc/90743e7298aca107ddaa0c202a4d3604e29bfeb6/library/core/src/cell.rs:958:9
     5: yjit::core::BlockRef::borrow_mut
               at /home/runner/work/ruby/ruby/src/yjit/src/core.rs:509:9
     6: yjit::core::BranchTarget::set_iseq
               at /home/runner/work/ruby/ruby/src/yjit/src/core.rs:379:46
     7: rb_yjit_iseq_update_references
               at /home/runner/work/ruby/ruby/src/yjit/src/core.rs:765:21
     8: rb_iseq_mark_and_update
               at ./../src/iseq.c:363:13
     9: gc_ref_update
               at ./../src/gc.c:10765:17
    10: gc_update_references
               at ./../src/gc.c:10799:13
    11: gc_compact_finish
               at ./../src/gc.c:5536:5
    12: gc_sweep_compact
               at ./../src/gc.c:8672:5
    13: gc_sweep
               at ./../src/gc.c:6196:9
    14: gc_marks
               at ./../src/gc.c:8735:9
    15: gc_start
               at ./../src/gc.c:9566:9
    16: garbage_collect
               at ./../src/gc.c:[94](https://github.com/ruby/ruby/actions/runs/3979122841/jobs/6821344008#step:19:99)47:15
    17: gc_start_internal
               at ./../src/gc.c:[98](https://github.com/ruby/ruby/actions/runs/3979122841/jobs/6821344008#step:19:103)67:5
    18: gc_verify_compaction_references
               at ./../src/gc.c:10[99](https://github.com/ruby/ruby/actions/runs/3979122841/jobs/6821344008#step:19:104)2:5
    19: <unknown>

https://github.com/ruby/ruby/actions/runs/3979122841/jobs/6821344008

[maximecb]

Will merge the fix, but would you also have a regression test that triggers this failure?

[k0kubun]

Will merge the fix, but would you also have a regression test that triggers this failure?

worked on that at ba482d0. I confirmed that this fails on #7168 (I need that change to avoid splitting a block on opt_plus)

[maximecb]

Thank you Kokubun! :)