tcg/optimize: Drop nb_oargs, nb_iargs locals

tcg/optimize: Drop nb_oargs, nb_iargs locals

Rather than try to keep these up-to-date across folding,
re-read nb_oargs at the end, after re-reading the opcode.

A couple of asserts need dropping, but that will take care
of itself as we split the function further.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>