clean up internal comments about float semantics

- remove an outdated FIXME
- add reference to floating-point semantics issue

Co-authored-by: Jubilee <workingjubilee@gmail.com>

tests/ui/numbers-arithmetic/issue-105626.rs

@@ -11,6 +11,7 @@ fn main() {
     assert_ne!((n as f64) as f32, n as f32);
 
     // FIXME: these assertions fail if only x87 is enabled
+    // see also https://github.com/rust-lang/rust/issues/114479
     assert_eq!(n as i64 as f32, r);
     assert_eq!(n as u64 as f32, r);
 }