RISC-V: Refine Phase 3 of VSETVL PASS

RISC-V: Refine Phase 3 of VSETVL PASS

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc (can_backward_propagate_p): Fix for null iter_bb.
	(vector_insn_info::set_demand_info): New function.
	(pass_vsetvl::emit_local_forward_vsetvls): Adjust for refinement of Phase 3.
	(pass_vsetvl::merge_successors): Ditto.
	(pass_vsetvl::compute_global_backward_infos): Ditto.
	(pass_vsetvl::backward_demand_fusion): Ditto.
	(pass_vsetvl::forward_demand_fusion): Ditto.
	(pass_vsetvl::demand_fusion): New function.
	(pass_vsetvl::lazy_vsetvl): Adjust for refinement of phase 3.
	* config/riscv/riscv-vsetvl.h: New function declaration.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-27.c: Update
	testcase.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-28.c: Ditto.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-45.c: Ditto.
	* gcc.target/riscv/rvv/vsetvl/vlmax_bb_prop-25.c: Ditto.
	* gcc.target/riscv/rvv/vsetvl/vlmax_bb_prop-26.c: Ditto.
	* gcc.target/riscv/rvv/vsetvl/vlmax_bb_prop-27.c: Ditto.
	* gcc.target/riscv/rvv/vsetvl/vlmax_bb_prop-28.c: Ditto.
	* gcc.target/riscv/rvv/vsetvl/vlmax_bb_prop-3.c: Ditto.
	* gcc.target/riscv/rvv/vsetvl/vlmax_conflict-7.c: Ditto.
	* gcc.target/riscv/rvv/vsetvl/vlmax_switch_vtype-12.c: Ditto.