[perl #74946] %h.delete() in right side of expression causes left side key missing

[Christian Bartolomaeus via RT]

This works now (using current syntax -- e.g. :delete):

$ perl6-m -e 'my %h = a => "b"; %h{"a"} = %h{"a"} :delete; say %h.perl;'
("a" => "b").hash

I added a test to S32-hash/delete-adverb.t with the following commit: 
https://github.com/perl6/roast/commit/96c87780f7