The CMPXCHG instruction has the following operands:
- AX (implicit, CompareValue): input and output
- destination operand (*Value): input and output
- source operand (ExchangeValue): input
The IA32 version of InternalSyncCompareExchange16() correctly marks
CompareValue as input/output, but it marks (*Value) only as input.
The X64 version of InternalSyncCompareExchange16() attempts to mark both
CompareValue and (*Value) as input/output, but it doesn't use the
appropriate constraints for either operand.
Fix these issues. Furthermore, prefer the short "+" constraint for I/O
operands over the <output-operand-number> constraint that can be applied
to the input instances of I/O operands.