tcg: Widen gen_insn_data to uint64_t

We already pass uint64_t to restore_state_to_opc; this changes all
of the other uses from insn_start through the encoding to decoding.

Reviewed-by: Anton Johansson <anjo@rev.ng>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>