RISC-V: Convert the ADD/ADDI to the compressed MV/LI if RS1 is zero.

	2020-02-19  Nelson Chu  <nelson.chu@sifive.com>
	gas/
	* testsuite/gas/riscv/c-add-addi.d: New testcase.
	* testsuite/gas/riscv/c-add-addi.s: Likewise.
	opcodes/
	* riscv-opc.c (riscv_opcodes): Convert add/addi to the compressed
	c.mv/c.li if rs1 is zero.

Change-Id: Id939b5e6db80d267a832545f3ffef7b9ba881f7d