[perl #131099] [LTA] is assoc('list') gives "MVMArray: Can't pop from an empty array"

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# New Ticket Created by  Zoffix Znet 
# Please include the string:  [perl #131099]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=131099 >


On prefix and postfix ops, using `is assoc('list')` causes an LTA error without 
any location.

I've no idea if it makes sense for them to be assoc list; originally, I came 
across this while trying to make a postfix op `is equiv(&[orelse])`:

    $ perl6 -e 'sub prefix:<♥> ($) is prec({:assoc("list")}) { }; ♥42'
    ===SORRY!===
    MVMArray: Can't pop from an empty array
    
    $ perl6 -e 'sub postfix:<♥> ($) is prec({:assoc("list")}) { }; 42♥'
    ===SORRY!===
    MVMArray: Can't pop from an empty array
    
    $ perl6 -e 'sub postfix:<♥> ($) is equiv(&[orelse]) { }; 42♥'
    ===SORRY!===
    MVMArray: Can't pop from an empty array
    
    $ perl6 -v
    This is Rakudo version 2017.03-20-g027aa54 built on MoarVM version 
2017.03-7-g029d121
    implementing Perl 6.c.

The error points to at gen/moar/stage2/NQPHLL.nqp:1099  
(/home/zoffix/.rakudobrew/moar-nom/install/share/nqp/lib/NQPHLL.moarvm:EXPR_reduce)

However, I don't know if this should be fixed by throwing when attempting to is 
assoc list these types of ops or what...