Even if the minimum number of elements is 1 and the length doesn't change,
we don't know what vscale is so we can't classify it as identity mask. Instead it
is a zero element splat.
For reverse, we shouldn't classify it as a reverse unless there are at least 2 elements
in the mask. This applies to both fixed and scalable vectors. For fixed vectors, a single
element would be an identity shuffle. For scalable vector it's a zero elt splat.
Reviewed By: sdesmalen, liaolucy
Differential Revision: https://reviews.llvm.org/D124655
39e63bd2d8