[AArch64][GlobalISel] Don't use explicit zero registers for compare results.

[AArch64][GlobalISel] Don't use explicit zero registers for compare results.

These cause problems for later optimizations, just using an unused vreg like
SelectionDAG generates better code in the end, and obviates the need for some
GISel specific flag optimizations.

Differential Revision: https://reviews.llvm.org/D89419