target/arm: Tidy conditions in handle_vec_simd_shri

target/arm: Tidy conditions in handle_vec_simd_shri

The (size > 3 && !is_q) condition is identical to the preceeding test
of bit 3 in immh; eliminate it.  For the benefit of Coverity, assert
that size is within the bounds we expect.

Fixes: Coverity CID1385846
Fixes: Coverity CID1385849
Fixes: Coverity CID1385852
Fixes: Coverity CID1385857
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20180501180455.11214-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>