RISC-V: Add compressed instruction hints, and a few misc cleanups.

	gas/
	* config/tc-riscv.c (risc_ip) <o>: Add comment.
	* testsuite/gas/riscv/c-nonzero-imm.d,
	* testsuite/gas/riscv/c-nonzero-imm.l,
	* testsuite/gas/riscv/c-nonzero-imm.s,
	* testsuite/gas/riscv/c-nonzero-reg.d,
	* testsuite/gas/riscv/c-nonzero-reg.l,
	* testsuite/gas/riscv/c-nonzero-reg.s,
	* testsuite/gas/riscv/c-zero-imm-64.d,
	* testsuite/gas/riscv/c-zero-imm-64.s,
	* testsuite/gas/riscv/c-zero-imm.d, testsuite/gas/riscv/c-zero-imm.s,
	* testsuite/gas/riscv/c-zero-reg.d,
	* testsuite/gas/riscv/c-zero-reg.s: New.

	opcodes/
	* riscv-opc.c (match_c_add_with_hint, match_c_lui_with_hint): New.
	(riscv_opcodes) <li>: Delete "d,0" line.  Change Cj to Co.
	<andi, and, add, addiw, addw, c.addi>: Change Cj to Co.
	<add>: Add explanatory comment for 4-operand add instruction.
	<c.nop>: Add support for immediate operand.
	<c.mv, c.add>: Use match_c_add_with_hint instead of match_c_add.
	<c.lui>: Use match_c_lui_with_hint instead of match_c_lui.
	<c.li, c.slli>: Use match_opcode instead of match_rd_nonzero.