[IR][DAG][RISCV] Allow scalable vector ISD::STRICT_FP_EXTEND and RISC-V...

[IR][DAG][RISCV] Allow scalable vector ISD::STRICT_FP_EXTEND and RISC-V supports for vector ISD::STRICT_FP_EXTEND.

The patch mainly does two things. The first is allowing scalable vector
ISD::STRICT_FP_EXTEND. The second is making RISC-V customized lower
strict_fpextend to riscv_strict_fpextend_vl, the strict version of
riscv_fpextend_vl.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D145548