Ensure we guard the value before we return

Otherwise you can end up not implicitly calling `to_ary`, which if it has side-effects will result in different behavior.

bootstraptest/test_yjit.rb

@@ -1479,3 +1479,18 @@ assert_equal '2', %q{
   expandarray_postarg
   expandarray_postarg
 }
+
+assert_equal '10', %q{
+  obj = Object.new
+  val = nil
+  obj.define_singleton_method(:to_ary) { val = 10; [] }
+
+  def expandarray_always_call_to_ary(object)
+    * = object
+  end
+
+  expandarray_always_call_to_ary(obj)
+  expandarray_always_call_to_ary(obj)
+
+  val
+}

yjit_codegen.c

@@ -774,16 +774,16 @@ gen_expandarray(jitstate_t* jit, ctx_t* ctx)
     rb_num_t num = (rb_num_t) jit_get_arg(jit, 0);
     x86opnd_t array_opnd = ctx_stack_pop(ctx, 1);
 
-    // If we don't actually want any values, then just return.
-    if (num == 0) {
-        return YJIT_KEEP_COMPILING;
-    }
-
     // Move the array from the stack into REG0 and check that it's an array.
     mov(cb, REG0, array_opnd);
     guard_object_is_heap(cb, REG0, ctx, COUNTED_EXIT(side_exit, expandarray_not_array));
     guard_object_is_array(cb, REG0, REG1, ctx, COUNTED_EXIT(side_exit, expandarray_not_array));
 
+    // If we don't actually want any values, then just return.
+    if (num == 0) {
+        return YJIT_KEEP_COMPILING;
+    }
+
     // Pull out the embed flag to check if it's an embedded array.
     x86opnd_t flags_opnd = member_opnd(REG0, struct RBasic, flags);
     mov(cb, REG1, flags_opnd);