creating a Buf with a shaped native array doesn't use the fast native array .new

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Migrated from rt.perl.org#131086 (status was 'new')

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[p6rt]

From @MasterDuke17

Some discussion here​:
https://irclog.perlgeek.de/moarvm/2017-04-01#i_14360267

my int @​i[10];
buf8.new(@​i);

The above code doesn't use the native array Buf.new candidate because it
doesn't understand native shaped array.