RISC-V: Use vector_length_operand instead of csr_operand in vsetvl patterns

This patch replaces csr_operand by vector_length_operand in the vsetvl
patterns.  This allows future changes in the vector code (i.e. in the
vector_length_operand predicate) without affecting scalar patterns that
use the csr_operand predicate.

gcc/ChangeLog:

	* config/riscv/vector.md:
	Use vector_length_operand for vsetvl patterns.

Co-authored-by: Jin Ma <jinma@linux.alibaba.com>
Co-authored-by: Xianmiao Qu <cooper.qu@linux.alibaba.com>
Co-authored-by: Christoph Müllner <christoph.muellner@vrull.eu>