target/i386: rewrite flags writeback for ADCX/ADOX

target/i386: rewrite flags writeback for ADCX/ADOX

Avoid using set_cc_op() in preparation for implementing APX; treat
CC_OP_EFLAGS similar to the case where we have the "opposite" cc_op
(CC_OP_ADOX for ADCX and CC_OP_ADCX for ADOX), except the resulting
cc_op is not CC_OP_ADCOX. This is written easily as two "if"s, whose
conditions are both false for CC_OP_EFLAGS, both true for CC_OP_ADCOX,
and one each true for CC_OP_ADCX/ADOX.

The new logic also makes it easy to drop usage of tmp0.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>