Don't double-count simd_shuffle promotion candidates

The proper attribute was added to `simd_shuffle*` in
rust-lang/stdarch#825. This caused `promote_consts` to double-count its
second argument when recording promotion candidates, which caused
the promotion candidate compatibility check to fail.

Author: ecstatic-morse

src/librustc_mir/transform/promote_consts.rs

@@ -199,6 +199,8 @@ impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
                             bb: location.block,
                             index: 2,
                         });
+
+                        return; // Don't double count `simd_shuffle` candidates
                     }
                 }