AArch64/ARM64: optimise vector selects & enable test

When performing a scalar comparison that feeds into a vector select,
it's actually better to do the comparison on the vector side: the
scalar route would be "CMP -> CSEL -> DUP", the vector is "CM -> DUP"
since the vector comparisons are all mask based.
[msan] Fix -fsanitize=memory -fno-integrated-as.