libdw: Simplify __libdw_visit_scopes' tag checks

libdw: Simplify __libdw_visit_scopes' tag checks

The former classify_die() was splitting tags into more classes than
actually needed.

The one place that used the "imported" die_class now just compares to
DW_TAG_imported_unit directly.

The recursion check was squashing "match", "match_inline", and "walk"
into the same action.  Now that uses the new may_have_scopes(), which
just returns true for all tags that had those classifications.

The net result has no functional change, but performs better.

Signed-off-by: Josh Stone <jistone@redhat.com>