[llvm] [RISCV][TTI] Implement vector costs for `llvm.fpto{u|s}i.sat()`. (PR #143655)

[Elvis Wang via llvm-commits]

ElvisWang123 wrote:

Ping @topperc :)

After #147657, the invalid cost for `llvm.fpto{u|s}i` is handled by basic TTI. 
This patch only  correct the cost of `llvm.fpto{u|s}i` for RISCV target.

https://github.com/llvm/llvm-project/pull/143655