target/arm: Use vector operations for saturation

target/arm: Use vector operations for saturation

For same-sign saturation, we have tcg vector operations.  We can
compute the QC bit by comparing the saturated value against the
unsaturated value.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190209033847.9014-12-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>