[RISCV] Break the (czero_eqz x, (setne x, 0)) -> x combine into 2 combines. (#90428)

We can think of this as two separate combines

(czero_eqz x, (setne y, 0)) -> (czero_eqz x, y)
and
(czero_eqz x, x) -> x

Similary the (czero_nez x, (seteq x, 0)) -> x combine can be broken into

(czero_nez x, (seteq y, 0)) -> (czero_eqz x, y)
and
(czero_eqz x, x) -> x

isel already does the (czero_eqz x, (setne y, 0)) -> (czero_eqz x, y)
and (czero_nez x, (seteq y, 0)) -> (czero_eqz x, y) combines, but doing
them early could expose other opportunities.