[CodeGen][SVE] Avoid scalarizing zero splat stores on scalable vectors.

Summary: Implemented in replaceZeroVectorStore(). Fixes several warnings in AArch64 SVE unit tests.

Reviewers: sdesmalen, kmclaughlin, dancgr, efriedma, each, andwar, rengolin

Reviewed By: sdesmalen

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80824