Improve sysretq checks.
Cherry-picked from the first commit of !225 (closed)
Fixes singlestepping after SYSRETQ, and simplifies SYSRETQ code by doing two cheap instructions rather than branching.
Cherry-picked from the first commit of !225 (closed)
Fixes singlestepping after SYSRETQ, and simplifies SYSRETQ code by doing two cheap instructions rather than branching.