https://issues.dlang.org/show_bug.cgi?id=23123

          Issue ID: 23123
           Summary: -vasm wrong result for cmpxchg16b
           Product: D
           Version: D2
          Hardware: x86_64
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: maxhaton@gmail.com

Seems to ignore the rex prefix.

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