Make all cases of statementlist loop eval to Nil

Previously, `loop { }` and `loop (; $++ < 3;) { }` would evaluate to
Nil, but `loop (my $i = 0; $i < 3; $i++) { }` would evaluate to an
`nqp::null` due to the oversight that is corrected in this commit. This
bug was made more visible when recent changes to return handling removed
the `nqp::null` mapping into Mu, resulting in the problem observed in
issue #2069. Since all other loop cases evaluated to Nil, this created a
discontinuity where one particular case of `loop` would instead evaluate
to Mu. Therefore, there was already a bug; the recent changes just made
it problematic in more cases.

src/Perl6/Actions.nqp

@@ -2227,7 +2227,12 @@ class Perl6::Actions is HLL::Actions does STDActions {
             $loop := QAST::Stmts.new( UNWANTED($<e1>.ast, 'statement_control/e1'), $loop, :node($/) );
         }
         my $sinkee := $loop[1];
-        $loop.annotate('statement_level', -> { UNWANTED($sinkee,'force loop') });
+        $loop.annotate('statement_level', -> {
+            UNWANTED($sinkee,'force loop');
+            if $<e1> {
+                $loop.push(QAST::WVal.new( :value($*W.find_symbol(['Nil'])) ));
+            }
+        });
         make $loop;
     }