RISC-V: Forbidden fuse vlmax vsetvl to DEMAND_NONZERO_AVL vsetvl

Hi,

This little patch fix the fail testcase
(gcc.target/riscv/rvv/autovec/gather-scatter/strided_load_run-1.c)
after apply this patch
(https://gcc.gnu.org/pipermail/gcc-patches/2023-August/627121.html).
The specific reason is that the vsetvl pass has bug and this patch
forbidden the fuse of this case. This patch needs to be committed
before that patch to work.

Best,
Lehua

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc (pass_vsetvl::backward_demand_fusion):
	Forbidden.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/gather-scatter/strided_load_run-1.c:
	Address failure due to uninitialized vtype register.