target-s390: Optimize ADDC/SUBB

target-s390: Optimize ADDC/SUBB

Giving the proper mask to disas_jcc allows us to generate an inline
comparison generating the carry/borrow with setcond.

In the very worst case, when we must use the external helper to compute
a value for CC, we generate (cc > 1) instead of (cc >> 1), which is only
very slightly slower on common cpus.

In the very best case, when the CC comes from a COMPARE insn and the
compiler is using ALCG with zero, everything folds out to become just
the setcond that the compiler wanted.

Signed-off-by: Richard Henderson <rth@twiddle.net>