RISC-V: Add support for addi that compresses to c.nop.

RISC-V: Add support for addi that compresses to c.nop.

	gas/
	* testsuite/gas/riscv/c-zero-imm.s: Test addi that compresses to c.nop.
	* testsuite/gas/riscv/c-zero-imm.d: Likewise.
	opcodes/
	* riscv-opc.c (match_c_nop): New.
	(riscv_opcodes) <addi>: Handle an addi that compresses to c.nop.