[ConstantFold][SVE] Fix constant folding for scalable vector unary operations.

Summary:
Similar to issue D71445. Scalable vector should not be evaluated element by element.
Add support to handle scalable vector UndefValue.

Reviewers: sdesmalen, efriedma, apazos, huntergr, willlovett

Reviewed By: efriedma

Subscribers: tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73678