[AArch64][GlobalISel] Don't write to WZR in non-flag-setting G_BRCOND case

We are avoiding writing to WZR just about everywhere else.

Also update the code to use MachineIRBuilder for the sake of consistency.

We also didn't have a GlobalISel testcase for this path, so add a simple one
now.

Differential Revision: https://reviews.llvm.org/D90626