[perl #79116] [BUG] Non-associative operators mistakingly allowed in reduction meta-op

[Christian Bartolomaeus via RT]

The output has changed, but it still doesn't die:

$ perl6 -e 'say [leg] <a b c>'
Less

I added a test (fudged 'todo') to S03-metaops/reduce.t with commit 
https://github.com/perl6/roast/commit/9078c34ec2