Don’t assume targs are contiguous for ‘my $x; my $y’

In commit 18c931a3, the padrange optimisation was prevented from mak-
ing this assumption for ‘my ($x,$y)’, but the assumption was still
there in the code that combines multiple statements into one.

This would lead to assertion failures (or, as of ce0d59f, crashes
under non-debugging builds) if a keyword plugin declined to handle
the second ‘my’, but only after creating a padop.

This fixes a regression from 5.16 affecting Devel::CallParser under
threaded builds.