ARM test failure client.drreg-test: "opnd_get_reg called on non-reg opnd"

[egrimley-arm]

Found on Debian 10, kernel 5.4.47, Cortex-A72 with a recent commit (9931511): ctest -V -R client.drreg-test

The test passed before e8fc651 (2021-05-27) but from a glance at the diff it seems fairly likely that that change just made the test more sensitive rather than break anything. The error with e8fc651 is different: drreg-test.dll.c:151: cannot unreserve register

(I'm currently seeing 49 test failures on the platform just described: see #3699. I'm creating a separate issue for this one as I have the additional information that I've just given but I don't have a fix.)