[RISCV] Add patterns of SET[U]LT_VI for STECC forms

This patch optmizes "li a0, 5
                     vmsgt[u].vx v10, v8, a0"
                 -> "vmsgt[u].vi v10, v8, 5"

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D118014