while (my $x ...) { ...; redo } shouldn't undef $x.

while (my $x ...) { ...; redo } shouldn't undef $x.
In the presence of 'my' in the conditional of a while(), until(),
or for(;;) loop, add an extra scope to the body so that redo
doesn't undef the lexical

p4raw-id: //depot/perl@24412