target/arm: Fix VLDRB/H/W for predicated elements

target/arm: Fix VLDRB/H/W for predicated elements

For vector loads, predicated elements are zeroed, instead of
retaining their previous values (as happens for most data
processing operations). This means we need to distinguish
"beat not executed due to ECI" (don't touch destination
element) from "beat executed but predicated out" (zero
destination element).

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