[IR][CostModel] A scalable vector shuffle can't be an identity or reverse shuffle.

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