[RISCV][SDAG] Improve codegen of select with constants if zicond is available (#82456)

This patch uses `add + czero.eqz/nez` to lower select with constants if
zicond is available.
```
(select c, c1, c2) -> (add (czero_nez c2 - c1, c), c1)
(select c, c1, c2) -> (add (czero_eqz c1 - c2, c), c2)
```
The above code sequence is suggested by [RISCV Optimization
Guide](https://riscv-optimization-guide-riseproject-c94355ae3e6872252baa952524.gitlab.io/riscv-optimization-guide.html#_avoid_branches_using_conditional_moves).