Also allow returning PassMode::ByValPair directly into an ssa var

Benchmark #1: ./raytracer_cg_clif_pre
  Time (mean ± σ):      8.251 s ±  0.021 s    [User: 8.245 s, System: 0.005 s]
  Range (min … max):    8.225 s …  8.292 s    10 runs

Benchmark #2: ./raytracer_cg_clif_post
  Time (mean ± σ):      8.206 s ±  0.043 s    [User: 8.199 s, System: 0.007 s]
  Range (min … max):    8.168 s …  8.279 s    10 runs

src/abi/returning.rs

@@ -10,9 +10,9 @@ pub(crate) fn can_return_to_ssa_var<'tcx>(
     dest_layout: TyAndLayout<'tcx>,
 ) -> bool {
     match get_pass_mode(tcx, dest_layout) {
-        PassMode::NoPass | PassMode::ByVal(_) => true,
-        // FIXME Make it possible to return ByValPair and ByRef to an ssa var.
-        PassMode::ByValPair(_, _) | PassMode::ByRef { size: _ } => false,
+        PassMode::NoPass | PassMode::ByVal(_) | PassMode::ByValPair(_, _) => true,
+        // FIXME Make it possible to return ByRef to an ssa var.
+        PassMode::ByRef { size: _ } => false,
     }
 }