KEEP + for loop = useless useless use warning

[zoffixznet]

The value has a potentially useful impact and IMO the warning should not be issued:

$ perl6 -e 'm: for ^5 { KEEP say "and"; 42 }'
WARNINGS for -e:
Useless use of constant integer 42 in sink context (line 1)
and
and
and
and
and

$ perl6 -e 'm: for ^5 { KEEP say "and"; Any }'
WARNINGS for -e:
Useless use of constant value Any in sink context (line 1)

$ perl6 -v
This is Rakudo version 2017.11-8-g7939014 built on MoarVM version 2017.11
implementing Perl 6.c.