Fixes a bug involving rep string expansion where drbbdup told drreg to ignore control flow even when it was present in the app block. Adds a guard on that setting based on a check for app control flow.
Adds a custom app for the drbbdup-emul-test which contains a zero-iter rep string, and augments the test client to reserve rcx on the movs only. This reproduces the #5398 (closed) crash, and is fixed by the changes here.
Fixes #5398 (closed)