[test] Move pr73894.ll to AArch64 directory and update the target triple (#84269)

pr73894.ll is failing on a number of non-AArch64 buildbots. I'm not
certain that this is a proper fix, but I think it's best to move the
test to the test/Transforms/LoopVectorize/AArch64/ directory and replace
the triple with one commonly used in that directory.

llvm#73894