[RISCV] Make SMIN/SMAX/UMIN/UMAX legal with Zbb extension.

This is the logically correct thing to do. But it generates worse
code for i32 umin/umax on the rv64 due to type legalize requesting
zext even though the arguments are sext. Maybe we can teach type
legalizer to use sext for umin/umax for RISCV.

It's also producing possibly worse code on i64 on RV32 since we
still end up with selects that become branches. But this seems
like something we could improve in type legalization or DAG combine.

Hopefully this makes D92095 work for RISCV with Zbb.