Introduce two types of fancy new helpers that will be reused a couple of
times

1. gen_gvec_fn_3: Call an existing tcg_gen_gvec_X function with 3
   parameters, simplifying parameter passing
2. gen_gvec128_3_i64: Call a function that performs 128 bit calculations
   using two 64 bit values per vector.

Luckily, for VECTOR ADD we already have everything we need.
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Signed-off-by: NDavid Hildenbrand <david@redhat.com>