clarify fixme waiting for a newer llvm version

Author: folkertdev

crates/core_arch/src/s390x/vector.rs

@@ -1009,7 +1009,8 @@ mod sealed {
     test_impl! { vec_roundc_f32 (a: vector_float) -> vector_float [nearbyint_v4f32,  "vector-enhancements-1" vfisb] }
     test_impl! { vec_roundc_f64 (a: vector_double) -> vector_double [nearbyint_v2f64, vfidb] }
 
-    // FIXME(llvm) roundeven does not yet lower to vfidb (but should in the future)
+    // FIXME(llvm) llvm trunk already lowers roundeven to vfidb, but rust does not use it yet
+    // use https://godbolt.org/z/cWq95fexe to check, and enable the instruction test when it works
     test_impl! { vec_round_f32 (a: vector_float) -> vector_float [roundeven_v4f32, _] }
     test_impl! { vec_round_f64 (a: vector_double) -> vector_double [roundeven_v2f64, _] }