ZJIT: Remove eager nil-fill of locals in JIT-to-JIT calls (ruby#16544)

The caller in gen_send_iseq_direct was eagerly writing Qnil to all
non-parameter local slots of the callee's frame before every JIT-to-JIT
call. This is unnecessary because compile_jit_entry_state already
initializes non-parameter locals to Const(Qnil) in the JIT entry block,
and these values are propagated to the target block via branch edges.
Before any non-leaf call (including eval/binding), gen_spill_locals
writes these nil values from the FrameState to the stack, ensuring
that eval can correctly read uninitialized locals as nil.

The nil-fill in function_stub_hit's prepare_for_exit is kept because
that path handles compilation failures where JIT code never runs.

Author: k0kubun

zjit/src/codegen.rs

@@ -1619,16 +1619,6 @@ fn gen_send_iseq_direct(
         0
     };
 
-    // Fill non-parameter locals with nil (they may be read by eval before being written)
-    let num_params = params.size.to_usize();
-    if local_size > num_params {
-        asm_comment!(asm, "initialize non-parameter locals to nil");
-        for local_idx in num_params..local_size {
-            let offset = local_size_and_idx_to_bp_offset(local_size, local_idx);
-            asm.store(Opnd::mem(64, SP, -offset * SIZEOF_VALUE_I32), Qnil.into());
-        }
-    }
-
     // Make a method call. The target address will be rewritten once compiled.
     let iseq_call = IseqCall::new(iseq, num_optionals_passed);
     let dummy_ptr = cb.get_write_ptr().raw_ptr(cb);
@@ -2876,7 +2866,7 @@ c_callable! {
             let pc = unsafe { rb_iseq_pc_at_idx(iseq, entry_insn_idxs[iseq_call.jit_entry_idx.to_usize()]) };
             unsafe { rb_set_cfp_pc(cfp, pc) };
 
-            // Successful JIT-to-JIT calls fill nils to non-parameter locals in generated code.
+            // JIT-to-JIT calls don't eagerly fill nils to non-parameter locals.
             // If we side-exit from function_stub_hit (before JIT code runs), we need to set them here.
             fn prepare_for_exit(iseq: IseqPtr, cfp: CfpPtr, sp: *mut VALUE, compile_error: &CompileError) {
                 unsafe {