[AArch64][GlobalISel] Legalize small G_TRUNC (#85625)

This is an alternative to #85610, that moreElement's small G_TRUNC
vectors to widen the vectors. It needs to disable one of the existing
Unmerge(Trunc(..)) combines, and some of the code is not as optimal as
it could be. I believe with some extra optimizations it could look
better (I was thinking combining trunc(buildvector) -> buildvector and
possibly improving buildvector lowering by generating
insert_vector_element earlier).