While writing into ivars of other types is too complicated to
realistically generate the ASM for it, we can at least provide
the ivar index as to not have to lookup the shape tree every
time.

```
compare-ruby: ruby 3.5.0dev (2025-08-29T07:25:05Z yjit-set-gen-ivar dc555a4) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-29T07:25:05Z yjit-set-gen-ivar dc555a4) +YJIT +PRISM [arm64-darwin24]

|                         |compare-ruby|built-ruby|
|:------------------------|-----------:|---------:|
|vm_ivar_set_on_instance  |      1.664k|    1.672k|
|                         |           -|     1.00x|
|vm_ivar_set_on_generic   |     172.193|   257.789|
|                         |           -|     1.50x|
|vm_ivar_set_on_class     |     184.145|   377.723|
|                         |           -|     2.05x|
```

The `vm_ivar_set_on_class` case is still noticeably slower
than `vm_ivar_set_on_instance` because `rb_ivar_set_at` performs
ractor checks. We could bypass them with `assume_single_ractor_mode`
to acheived ~2.2x the baseline.

Then it's also noticeably slower because the benchmark sets immediate
and YJIT skip triggering write barriers for those, but `RB_OBJ_WRITE`
always does. If we skip the write barrier we end up at ~3.5x the baseline.

```
compare-ruby: ruby 3.5.0dev (2025-08-29T07:25:05Z yjit-set-gen-ivar dc555a4) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-29T07:25:05Z yjit-set-gen-ivar dc555a4) +YJIT +PRISM [arm64-darwin24]

|                         |compare-ruby|built-ruby|
|:------------------------|-----------:|---------:|
|vm_ivar_set_on_instance  |      1.671k|    1.666k|
|                         |       1.00x|         -|
|vm_ivar_set_on_generic   |     172.174|   348.817|
|                         |           -|     2.03x|
|vm_ivar_set_on_class     |     176.337|   611.242|
|                         |           -|     3.47x|
```

If the benchmark was modified to set an heap value rather than
an immediate the T_OBJECT case would be much closer to the other
two:

```
compare-ruby: ruby 3.5.0dev (2025-08-29T07:25:05Z yjit-set-gen-ivar dc555a4) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-29T12:42:23Z yjit-set-gen-ivar ae7017f86d) +YJIT +PRISM [arm64-darwin24]

|                         |compare-ruby|built-ruby|
|:------------------------|-----------:|---------:|
|vm_ivar_set_on_instance  |     442.883|   442.278|
|                         |       1.00x|         -|
|vm_ivar_set_on_generic   |     165.336|   243.261|
|                         |           -|     1.47x|
|vm_ivar_set_on_class     |     145.395|   345.946|
|                         |           -|     2.38x|
```