[DAG][RISCV] Allow scalable vector ISD::STRICT_FP_ROUND and support vector...

[DAG][RISCV] Allow scalable vector ISD::STRICT_FP_ROUND and support vector ISD::STRICT_FP_ROUND for RISC-V.

The patch customized lower vector type ISD::STRICT_FP_ROUND to RISCVISD::STRICT_FP_ROUND.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D147113