Static optimizer breaks meta-assign to Array/Hashes returned from methods

[zoffixznet]

$ perl6 -e 'class Foo { has @.things is rw }; my $o := Foo.new; dd $o.things += <a b c>'
Cannot assign to an immutable value
  in block <unit> at -e line 1

$ perl6 --optimize=off -e 'class Foo { has @.things is rw }; my $o := Foo.new; dd $o.things += <a b c>'
Array @!things = [3]

$ perl6 -v
This is Rakudo version 2018.05-54-g148d7c5 built on MoarVM version 2018.05-57-g89dee3b
implementing Perl 6.c.