Exhaustivity check slowdown after enum exhaustivity added

[nicolasstucki]

In the benchmarks for exhaustivity V we can see that the performance went from 1.5 to 2.3 in #2197.

We should explore if this slowdown is justified and see if we can improve it.

[nicolasstucki]

Issue found in liufengyun/bench#11

[nicolasstucki]

@liufengyun do you have some more insight?

[allanrenucci]

@nicolasstucki Can we close this? The performance are back to 1.5