[RISCV] Optimize x <s -1 ? x : -1. Improve x >u 1 ? x : 1.

Similar to D132211, we can optimize x <s -1 ? x : -1 -> x <s 0 ? x : -1

Also improve the unsigned case from D132211 to use x != 0 which
will give a bnez instruction which might be compressible.

Differential Revision: https://reviews.llvm.org/D132252