[RISCV] Replace ISD::FP_EXTEND and ISD::FP_ROUND with RVV VL op.

This patch tries to solve the incoordination between the direct and intermediate  cast caused by D123975.
This patch replaces ISD::FP_EXTEND and ISD::FP_ROUND with RVV VL op in the lowering of FP scalable vector direct cast to unify with the intermediate cast.
And it also changes the FP widenning pattern with the VL op.

Differential Revision: https://reviews.llvm.org/D125364