target/arm: Tidy do_ldrq

Use tcg_constant_i32 for passing the simd descriptor,
as this hashed value does not need to be freed.
Rename dofs to doff to match poff.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210525010358.152808-78-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>