arm64: Work around a clang/LLVM misoptimization
arm64: Work around a clang/LLVM misoptimization

This avoids a misoptimization in clang,
https://bugs.llvm.org/show_bug.cgi?id=39550, where the root cause
has been around for a number of years, but a change in LLVM 6.0
allowed for better optimizations, exposing this bug. This bug is
on good track to be fixed in LLVM for the 8.0 release and hopefully
also for backporting into 7.0.1. It is however present in 6.0, 6.0.1
and 7.0, and other downstream users such as Xcode 10.0/10.1.

videolan/dav1d