YJIT: Add missing local variable type update for fallback setlocal blocks

Previously, the chain_depth>0 version of setlocal blocks did not
update the type of the local variable in the context. This can leave
the context with stale type information and trigger panics like in
[Bug #21772] or lead to miscompilation.

To trigger the issue, YJIT needs to see the same ISEQ before and after
environment escape and have tracked type info before the escape. To
trigger in ISEQs that do not send with a block, it probably requires
Kernel#binding or the use of include/ruby/debug.h APIs.

bootstraptest/test_yjit.rb

@@ -5416,3 +5416,22 @@ assert_equal 'foo', %{
 
   10.times.map { foo.dup }.last
 }
+
+# regression test for [Bug #21772]
+# local variable type tracking desync
+assert_normal_exit %q{
+  def some_method = 0
+
+  def test_body(key)
+    some_method
+    key = key.to_s # setting of local relevant
+
+    key == "symbol"
+  end
+
+  def jit_caller = test_body("session_id")
+
+  jit_caller # first iteration, non-escaped environment
+  alias some_method binding # induce environment escape
+  test_body(:symbol)
+}

yjit/src/codegen.rs

@@ -2518,6 +2518,7 @@ fn gen_setlocal_generic(
     ep_offset: u32,
     level: u32,
 ) -> Option<CodegenStatus> {
+    // Post condition: The type of of the set local is updated in the Context.
     let value_type = asm.ctx.get_opnd_type(StackOpnd(0));
 
     // Fallback because of write barrier
@@ -2539,6 +2540,11 @@ fn gen_setlocal_generic(
         );
         asm.stack_pop(1);
 
+        // Set local type in the context
+        if level == 0 {
+            let local_idx = ep_offset_to_local_idx(jit.get_iseq(), ep_offset).as_usize();
+            asm.ctx.set_local_type(local_idx, value_type);
+        }
         return Some(KeepCompiling);
     }
 
@@ -2591,6 +2597,7 @@ fn gen_setlocal_generic(
         );
     }
 
+    // Set local type in the context
     if level == 0 {
         let local_idx = ep_offset_to_local_idx(jit.get_iseq(), ep_offset).as_usize();
         asm.ctx.set_local_type(local_idx, value_type);