YJIT: Fix warn(unused_assignments) in test_gen_opt_reverse

Previously:

```
warning: value assigned to `value_array` is never read
     --> yjit/src/codegen.rs:11446:9
      |
11446 |         value_array[1] = 4;
      |         ^^^^^^^^^^^^^^^^^^
      |
      = help: maybe it is overwritten before being read?
      = note: `#[warn(unused_assignments)]` (part of `#[warn(unused)]`) on by default
```

It can't see the alias through `jit.pc`!

Author: XrXr

yjit/src/codegen.rs

@@ -11429,9 +11429,8 @@ mod tests {
         asm.stack_push(Type::Flonum);
         asm.stack_push(Type::CString);
 
-        let mut value_array: [u64; 2] = [0, 3];
-        let pc: *mut VALUE = &mut value_array as *mut u64 as *mut VALUE;
-        jit.pc = pc;
+        let mut fake_encoded_iseq: [VALUE; 2] = [VALUE(0), VALUE(3)];
+        jit.pc = &mut fake_encoded_iseq[0];
 
         let mut status = gen_opt_reverse(&mut jit, &mut asm);
 
@@ -11442,8 +11441,9 @@ mod tests {
         assert_eq!(Type::Fixnum, asm.ctx.get_opnd_type(StackOpnd(0)));
 
         // Try again with an even number of elements.
+        fake_encoded_iseq[1] = VALUE(4);
+        let _ = fake_encoded_iseq;
         asm.stack_push(Type::Nil);
-        value_array[1] = 4;
         status = gen_opt_reverse(&mut jit, &mut asm);
 
         assert_eq!(status, Some(KeepCompiling));