[AArch64] Match ZIP and UZP starting from undef elements. (#89578)

In case the first element of a zip/uzp mask is undef, the isZIPMask and
isUZPMask functions have a 50% chance of picking the wrong
"WhichResult", meaning they don't match a zip/uzp where they could. This
patch alters the matching code to first check for the first non-undef
element, to try and get WhichResult correct.