Fix spurious useless-use warnings in metaop+metaassign op

Fixes RT#131251: https://rt.perl.org/Ticket/Display.html?id=131251

We check if the meta's op has is_pure annotation and cry if
it does, but the use isn't useless if our meta is a metassign.

N.B.: the check for metaassign can probably be moved up to the outer
conditional, but IIUC it'd make it too wide without is-from-core check.

src/Perl6/Actions.nqp

@@ -350,7 +350,7 @@ sub unwanted($ast, $by) {
                         $op := nqp::substr($op, 0, $t) if $t > 0;
                         my $purity := 0;
                         if $node0[0].ann('is_pure') {
-                            $purity := 1;
+                            $purity := 1 unless $node0.name eq '&METAOP_ASSIGN';
                         }
                         else {
                             my $subname := $node0[0].name;

t/05-messages/10-warnings.t

@@ -2,7 +2,7 @@ use lib <t/packages/>;
 use Test;
 use Test::Helpers;
 
-plan 4;
+plan 5;
 
 subtest 'Supply.interval with negative value warns' => {
     plan 2;
@@ -67,4 +67,7 @@ is-run ｢
     print postcircumfix:<[ ]>(<foo bar ber>, 1)
 ｣, :out<bar>, 'no spurious warnings when invoking colonpaired routine';
 
+is-run ｢my $a; $a [R~]= "b"; $a [Z~]= "b"; $a [X~]= "b"｣,
+    'metaops + metaassign op do not produce spurious warnings';
+
 # vim: ft=perl6 expandtab sw=4