[RISCV] Support (truncate (smin (smax X, C1), C2)) for vnclipu in combineTruncToVnclip. (#93756)

If the smax removed all negative numbers, then we can treat the smin
like a umin.

If the smin and smax are in the other order we can swap them and use a
vnclipu as long as the smax constant is smaller than the smin constant.

This is based on similar code from X86's detectUSatPattern.