[x86] Correct the operand structure of the ADOX instruction.

This also moves to define it in the same way as ADCX which seems to use
constraints a bit better.

This is pulled out of the review for reducing the use of popf for
restoring EFLAGS, but is independent. There are still more problems with
our definitions for these instructions that Craig is going to look at
but this is at least less broken and he can start from this to improve
them more fully.

Thanks to Craig for the review here.

llvm-svn: 328945