[RISCV] Select unmasked integer setcc insts via ISel post-process

This patch has no effect on the generated code, whilst mitigating the
increase in ISel table size caused by the recent addition of masked
patterns.

I aim to do the same for floating-point patterns once D123051 lands,
giving us a reason to use masked floating-point patterns.

Reviewed By: arcbbb

Differential Revision: https://reviews.llvm.org/D123217