[perl #126001] Blockless LAST phaser puts Mu in the iteration variable

[Christian Bartolomaeus via RT]

Behaviour for Moar is unchanged. The code works as expected on JVM:

$ perl6-j -e 'for 1, 2 { LAST say $_ }'
2

I added a test to S04-phasers/in-loop.t with commit 
https://github.com/perl6/roast/commit/a1c52bacd3

BTW, this ticket looks related to the failing tests from 
https://rt.perl.org/Ticket/Display.html?id=121722