RISC-V: Fix ICE for avl_single-86/avl_single-88/avl_single-90

RISC-V: Fix ICE for avl_single-86/avl_single-88/avl_single-90

If prop is demand of vsetvl instruction and reaching doesn't demand
AVL. We don't backward propagate since vsetvl instruction has no
side effects.

FAIL: gcc.target/riscv/rvv/vsetvl/avl_single-86.c  -Og -g  (internal
compiler error: Segmentation fault)
FAIL: gcc.target/riscv/rvv/vsetvl/avl_single-86.c  -Og -g  (test for
excess errors)
FAIL: gcc.target/riscv/rvv/vsetvl/avl_single-88.c  -Og -g  (internal
compiler error: Segmentation fault)
FAIL: gcc.target/riscv/rvv/vsetvl/avl_single-88.c  -Og -g  (test for
excess errors)
FAIL: gcc.target/riscv/rvv/vsetvl/avl_single-90.c  -Og -g  (internal
compiler error: Segmentation fault)
FAIL: gcc.target/riscv/rvv/vsetvl/avl_single-90.c  -Og -g  (test for
excess errors)

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc (reg_available_p): Fix bug.
	(pass_vsetvl::backward_demand_fusion): Ditto.