[InstCombine] Support negation of scalable-vector splats

[InstCombine] Support negation of scalable-vector splats

This patch is an extension of D103421. It allows the InstCombiner to
generate the negated form of integer scalable-vector splats. It can
technically handle fixed-length vectors too but those are completely
covered by the preceding logic.

This enables extra combining opportunities for scalable vector types.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D103801