Created by: egrimley
Make instr_encode_arch() return NULL, without an assertion failure, if no encoding is found. The failure to encode might indicate an internal error, but this function is also called from the API, whose documented behaviour is to return NULL.
Also, on AArch64, move the code to handle OP_xx from codec.c to encode.c because if encode_common() accepts OP_xx then we cannot recognise a failure to encode from its uint return value.
Change-Id: I38f582c30481bb447c0a079ef12a5a80562f7176