target-i386: introduce gen_prepare_cc

target-i386: introduce gen_prepare_cc

This makes the i386 front-end able to create CCPrepare structs for all
condition, not just those that come from a single flag.  In particular,
JCC_L and JCC_LE can be optimized because gen_prepare_cc is not forced
to return a result in bit 0 (unlike gen_setcc_slow).

However, for now the slow jcc operations will still go through CC
computation in a single-bit temporary, followed by a brcond if the
temporary is nonzero.

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