tcg: Sink def, nb_iargs, nb_oargs loads in liveness_pass_1

tcg: Sink def, nb_iargs, nb_oargs loads in liveness_pass_1

Sink the sets of the def, nb_iargs, nb_oargs variables to
the default and do_not_remove labels.  They're not really
needed beforehand, and it avoids preceding code from having
to keep them up-to-date.  Note that def had *not* been kept
up-to-date; thankfully only def->flags had been used and
those bits were constant between opcode changes.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>