The condition (t2 == 0) never becomes true because the zero value
(i.e., E_NONE) is only used as a dummy type for prevtoken. It can
be passed to t1, but not to t2.

The caller of this function only checks expr_compare_type() > 0.
Therefore, the distinction between 0 and -1 is unnecessary.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>