ZJIT: unify `hir::Insn::{Send,SendWithoutBlock}` (GH-16143)

After GH-15911, we largely apply the same treatment to these two
variants everywhere. In `type_specialize`, there are two ~100 LOC
match arms that shared 90% of the code. Unifying these allowed the
code to be shared more easily.

---

**Note:**

In the interest of preserving existing semantics and keeping the
diff small for this initial PR, `YARVINSN_send` can currently
produce:

```rs
Insn::Send { blockiseq: Some(null_ptr), .. }
```

Either way, that can be done in a follow-up PR.

---

Follow-up tasks:

1. Unify `reduce_send_to_ccall`/`reduce_send_without_block_to_ccall`

2. Address the `Some(null_ptr)` situation above (discussed on PR)

3. Rename/merge the stats/counters/fallback reasons, if desired

Partially addresses Shopify/ruby#941 (TODO: `optimize_c_calls`)

[alan: rewrote commit message]
Reviewed-by: Alan Wu <XrXr@users.noreply.github.com>

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