target/i386: optimize CX handling in repeated string operations

target/i386: optimize CX handling in repeated string operations

In a repeated string operation, CX/ECX will be decremented until it
is 0 but never underflow.  Use this observation to avoid a deposit or
zero-extend operation if the address size of the operation is smaller
than MO_TL.

As in the previous patch, the patch is structured to include some
preparatory work for subsequent changes.  In particular, introducing
cx_next prepares for when ECX will be decremented *before* calling
fn(s, ot), and therefore cannot yet be written back to cpu_regs.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Link: https://lore.kernel.org/r/20241215090613.89588-11-pbonzini@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>