target/i386: tcg: use cout to commonize add/adc/sub/sbb cases

target/i386: tcg: use cout to commonize add/adc/sub/sbb cases

Use the carry-out vector as the basis to compute AF, CF and OF.  The cost
is pretty much the same, because the carry-out is just four boolean
operations, and the code is much smaller because add/adc/sub/sbb now
share most of it.

A similar algorithm to what is used in target/i386/emulate can also be
used for APX, in order to build the result of CCMP/CTEST with a new CC_OP_*.
CCMP needs to place into the flags from either a subtraction or a constant
value; CTEST likewise place into the flags either an AND or a constant
value.  The new CC_OP for CCMP and CTEST would store for a successful
predcate:

- in DST and SRC2, the result of the operation;

- in SRC, a carry-out vector for CCMP or zero for CTEST;

If the default flag value is used, DST/SRC/SRC2 can be filled with
constants:

- in DST the negated ZF;

- in SRC's top 2 bits, a value that results in the desired OF and CF;

- in SRC2 a suitable value (any of 0/1/~0/~1) that can be used
  instead of DST to compute the desired SF and PF.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>