RISC-V: Bugfix for riscv-vsetvl pass.

This patch comes from part of below change, which locate one bug of rvv
vsetvel pass when auto-vectorization.

https://gcc.gnu.org/pipermail/gcc-patches/2023-July/624523.html



Unforunately, It is not easy to reproduce this bug by intrinsic APIs
but it is worth to backport to GCC 13.

Signed-off-by: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc (gen_vsetvl_pat): Add vl parameter.
	(change_vsetvl_insn): Ditto.
	(change_insn): Add validate change as well as assert.
	(pass_vsetvl::backward_demand_fusion): Allow forward.