Due to the design of the disassembler, the immediate is not
known during decoding of the opcode; so to handle compressed
encodings with reserved immediate values (non-zero), we need
to add an additional check during decompression to match
reserved encodings with zero immediates and translate them
into the illegal instruction.

The following compressed opcodes have reserved encodings with
zero immediates: c.addi4spn, c.addi, c.lui, c.addi16sp, c.srli,
c.srai, c.andi and c.slli

Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
[Palmer: Broke long lines]
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>