RISC-V: compress "addi d,CV,z" to "c.mv d,CV"

RISC-V: compress "addi d,CV,z" to "c.mv d,CV"

PR 27732
gas/
	* testsuite/gas/riscv/c-zero-imm.d: Compress addi a4,a4,0 to
	c.mv a4,a4.
	* testsuite/gas/riscv/c-zero-imm.s: Likewise.
opcodes/
	* riscv-opc.c (riscv_opcodes): New insn alias for addi. Compress
	"addi d,CV,z" to "c.mv d,CV".