RISC-V: Remove dirty_pat since it is redundant

RISC-V: Remove dirty_pat since it is redundant

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc (vector_insn_info::operator==): Remove dirty_pat.
	(vector_insn_info::merge): Ditto.
	(vector_insn_info::dump): Ditto.
	(pass_vsetvl::merge_successors): Ditto.
	(pass_vsetvl::backward_demand_fusion): Ditto.
	(pass_vsetvl::forward_demand_fusion): Ditto.
	(pass_vsetvl::commit_vsetvls): Ditto.
	* config/riscv/riscv-vsetvl.h: Ditto.