[AArch64][GlobalISel] When generating SUBS for compares, don't write to wzr/xzr.

Although writing to wzr/xzr is correct since we don't care about the result
of the sub, only the flags, doing so causes tail merge blocks to fail.

Writing to an unused virtual register instead allows the optimization to fire,
improving performance significantly on 256.bzip2.

Differential Revision: https://reviews.llvm.org/D80460