[AArch64LoadStoreOptimizer] Don't treat write to XZR/WZR as a clobber.

Summary:
When searching for load/store instructions to pair/merge don't treat
writes to WZR/XZR as clobbers since they don't change the value read
from WZR/XZR (which is always 0).

Reviewers: mcrosier, junbuml, jmolloy, t.p.northover

Subscribers: aemerson, llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D26921

llvm-svn: 287592