target-arm/translate-a64.c: Remove dead ?: in disas_simd_3same_int()

In disas_simd_3same_int(), none of the instructions permit is_q
to be false with size == 3 (this would be a vector operation with
a one-element vector, and the instruction set encodes those as
scalar operations). Replace the always-true ?: check with an
assert.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 1402171881-14343-3-git-send-email-peter.maydell@linaro.org
