[perl #99732] [BUG] Rakudo doesn't like .++

[Christian Bartolomaeus via RT]

I added a test to S03-operators/autoincrement.t with the following commit: 
https://github.com/perl6/roast/commit/3164e55750

(I didn't test the staff with $_='aaaa' though, but only incremented $_=4 with 
.++ once.)