This unifies 5 copies of checks for supported vector size,
and in the process fixes a missing check in tcg_gen_gvec_2s.

This lead to an assertion failure for 64-bit vector multiply,
which is not available in the AVX instruction set.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>