Remove redundant union check in `KnownPanicsLint const prop

because we are already marking unions `NoPropagation` in
`CanConstProp::check()`. That is enough to prevent any attempts
at const propagating unions and this second check is not needed.

Also improve a comment in `CanConstProp::check()`.

compiler/rustc_mir_transform/src/known_panics_lint.rs

@@ -583,33 +583,21 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                 val.into()
             }
 
-            Aggregate(ref kind, ref fields) => {
-                // Do not const prop union fields as they can be
-                // made to produce values that don't match their
-                // underlying layout's type (see ICE #121534).
-                // If the last element of the `Adt` tuple
-                // is `Some` it indicates the ADT is a union
-                if let AggregateKind::Adt(_, _, _, _, Some(_)) = **kind {
-                    return None;
-                };
-                Value::Aggregate {
-                    fields: fields
-                        .iter()
-                        .map(|field| {
-                            self.eval_operand(field).map_or(Value::Uninit, Value::Immediate)
-                        })
-                        .collect(),
-                    variant: match **kind {
-                        AggregateKind::Adt(_, variant, _, _, _) => variant,
-                        AggregateKind::Array(_)
-                        | AggregateKind::Tuple
-                        | AggregateKind::RawPtr(_, _)
-                        | AggregateKind::Closure(_, _)
-                        | AggregateKind::Coroutine(_, _)
-                        | AggregateKind::CoroutineClosure(_, _) => VariantIdx::ZERO,
-                    },
-                }
-            }
+            Aggregate(ref kind, ref fields) => Value::Aggregate {
+                fields: fields
+                    .iter()
+                    .map(|field| self.eval_operand(field).map_or(Value::Uninit, Value::Immediate))
+                    .collect(),
+                variant: match **kind {
+                    AggregateKind::Adt(_, variant, _, _, _) => variant,
+                    AggregateKind::Array(_)
+                    | AggregateKind::Tuple
+                    | AggregateKind::RawPtr(_, _)
+                    | AggregateKind::Closure(_, _)
+                    | AggregateKind::Coroutine(_, _)
+                    | AggregateKind::CoroutineClosure(_, _) => VariantIdx::ZERO,
+                },
+            },
 
             Repeat(ref op, n) => {
                 trace!(?op, ?n);
@@ -897,8 +885,8 @@ impl CanConstProp {
         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
             let ty = body.local_decls[local].ty;
             if ty.is_union() {
-                // Do not const prop unions as they can
-                // ICE during layout calc
+                // Unions are incompatible with const prop because
+                // Rust has no conpcept of the active variant of a union
                 *val = ConstPropMode::NoPropagation;
             } else {
                 match tcx.layout_of(param_env.and(ty)) {