RISC-V: Refactor riscv_implied_info_t to make it able to handle conditional implicati...

RISC-V: Refactor riscv_implied_info_t to make it able to handle conditional implication [NFC]

RISC-V ISA implication rules become little bit complicated than before,
it may come with condition, so this commit extend the capability of
riscv_implied_info_t, also make it more...C++ize.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_implied_predicator_t): New.
	(riscv_implied_info_t::riscv_implied_info_t): New.
	(riscv_implied_info_t::match): New.
	(riscv_implied_info): New entry for zcf.
	(riscv_subset_list::handle_implied_ext): Use
	riscv_implied_info_t::match.
	(riscv_subset_list::check_implied_ext): Ditto.
	(riscv_subset_list::handle_combine_ext): Ditto.
	(riscv_subset_list::parse): Move zcf implication handling to
	riscv_implied_infos.