target/arm: Factor out mve_eci_mask()

target/arm: Factor out mve_eci_mask()

In some situations we need a mask telling us which parts of the
vector correspond to beats that are not being executed because of
ECI, separately from the combined "which bytes are predicated away"
mask.  Factor this mask calculation out of mve_element_mask() into
its own function.

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