tcg: Increase tcg_out_dupi_vec immediate to int64_t

While we don't store more than tcg_target_long in TCGTemp,
we shouldn't be limited to that for code generation.  We will
be able to use this for INDEX_op_dup2_vec with 2 constants.

Also pass along the minimal vece that may be said to apply
to the constant.  This allows some simplification in the
various backends.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>