kconfig: refactor expr_eliminate_dups()

Currently, expr_eliminate_dups() passes two identical pointers down to
expr_eliminate_dups1(), which later skips processing identical leaves.

This approach is somewhat tricky and, more importantly, it will not work
with the refactoring made in the next commit.

This commit slightly changes the recursion logic; it deduplicates both
the left and right arms, and then passes them to expr_eliminate_dups1().
expr_eliminate_dups() should produce the same result.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>