in LDSTCompUnit testing of sign_extend is needed.
commit 63c8560011b75345a0fd49427b63cd4040cab89d Author: Luke Kenneth Casson Leighton <lkcl@lkcl.net> Date: Wed Sep 2 17:48:17 2020 +0100 sign-extend lhax needs 16-64, separate from lwax which is 32-64