x86emul: reduce CMPXCHG{8,16}B footprint and casting

x86emul: reduce CMPXCHG{8,16}B footprint and casting

Re-use an existing stack variable (reducing stack footprint, which also
results in smaller code due to some stack accesses no longer needing a
32-bit displacement), at once using a union instead of casts. Also
switch to rex_prefix based conditionals instead of op_bytes ones.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>