target-i386: introduce CCPrepare

Introduce a struct that describes how to build a *cond operation
that checks for a given x86 condition code.  For now, just change
gen_compute_eflags_* to return the new struct, generate code for
the CCPrepare struct, and go on as before.

[rth: Use ctz with the proper width rather than ffs.]

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
