[RISCV] Support vector SET[U]LT and SET[U]GE with splatted immediates

This patch adds more optimized codegen for the above SETCC forms,
by matching the '.vi' vector forms when the immediate is a 5-bit signed
immediate plus 1. The immediate can be decremented and the corresponding
SET[U]LE or SET[U]GT forms can be matched.

This work was left as a TODO from D94168.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D100096