target-i386: do not compute eflags multiple times consecutively

target-i386: do not compute eflags multiple times consecutively

After calling gen_compute_eflags, leave the computed value in cc_reg_src
and set cc_op to CC_OP_EFLAGS.  The next few patches will remove anyway
most calls to gen_compute_eflags.

As a result of this change it is more natural to remove the register
argument from gen_compute_eflags and change all the callers.

Reviewed-by: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>