target/i386: cleanup cc_op changes for REP/REPZ/REPNZ

gen_update_cc_op must be called before control flow splits.  Do it
where the jump on ECX!=0 is translated.

On the other hand, remove the call before gen_jcc1, which takes care of
it already, and explain why REPZ/REPNZ need not use CC_OP_DYNAMIC---the
translation block ends before any control-flow-dependent cc_op could
be observed.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>