[AArch64][SVE][InstCombine] last{a,b} of a splat vector

Replace last{a,b}(splat(X)) with X, irrespective of the predicate.

Patch by/Committing on behalf of: Usman Nadeem (mnadeem)

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D105520