target/microblaze: Convert dec_add to decodetree

target/microblaze: Convert dec_add to decodetree

Adds infrastrucure for translation of instructions, which could
not be added before their first use.  Cache a temporary which
represents r0 as the immediate 0 value, or a sink.

Move the special case of opcode_0_illegal from old_decode()
into decodetree as well, lest this get interpreted as add.

Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>