YJIT: Fixes failure reported by rails for opt+splat+rest (#7727)

bootstraptest/test_yjit.rb

@@ -3933,3 +3933,17 @@ assert_equal '[true, true, true, true, true]', %q{
   end
   calling_my_func
 }
+
+# Regresssion test: rest and optional and splat
+assert_equal 'true', %q{
+  def my_func(base=nil, *args)
+    [base, args]
+  end
+
+  def calling_my_func
+    array = []
+    my_func(:base, :rest1, *array) == [:base, [:rest1]]
+  end
+
+  calling_my_func
+}

yjit/src/codegen.rs

@@ -5855,7 +5855,7 @@ fn gen_send_iseq(
 
                 // If we have more arguments than required, we need to prepend
                 // the items from the stack onto the array.
-                let diff = (non_rest_arg_count - required_num + opts_filled_with_splat.unwrap_or(0)) as u32;
+                let diff = (non_rest_arg_count - (required_num + opts_filled_with_splat.unwrap_or(0))) as u32;
 
                 // diff is >0 so no need to worry about null pointer
                 asm.comment("load pointer to array elements");