[AArch64] Fix bug in store of vector 0 DAGCombine.

[AArch64] Fix bug in store of vector 0 DAGCombine.

Summary:
Avoid using XZR/WZR directly as operands to split stores of zero
vectors.  Doing so can lead to the XZR/WZR being used by an instruction
that doesn't allow it (e.g. add).

Fixes bug 34674.

Reviewers: t.p.northover, efriedma, MatzeB

Subscribers: aemerson, rengolin, javed.absar, mcrosier, eraman, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D38146

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313916 91177308-0d34-0410-b5e6-96231b3b80d8