target/i386: pull cc_op update to callers of gen_jmp_rel{,_csize}

target/i386: pull cc_op update to callers of gen_jmp_rel{,_csize}

gen_update_cc_op must be called before control flow splits.  Doing it
in gen_jmp_rel{,_csize} may hide bugs, instead assert that cc_op is
clean---even if that means a few more calls to gen_update_cc_op().

With this new invariant, setting cc_op to CC_OP_DYNAMIC is unnecessary
since the caller should have done it.

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