Showing that only non-volatile registers are reliably saved. Volatile
registers are only saved by NtGetContextThread whenever it interrupts
a thread in user space, and are otherwise returned from some previous,
possibly outdated, state.
@jacek Unless I'm missing something, I think this shows that we do not
have to save the full context in syscalls in general, and instead only
the non-volatile XMM registers?
NtGetContextThread syscall still probably needs to save the full context
and it should probably be using a specific code path.
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https://gitlab.winehq.org/wine/wine/-/merge_requests/1244