[AArch64][GlobalISel] Select Bcc when it's better than TB(N)Z

Instead of falling back to selecting TB(N)Z when we fail to select an
optimized compare against 0, select Bcc instead.

Also simplify selectCompareBranch a little while we're here, because the logic
was kind of hard to follow.

At -O0, this is a 0.1% geomean code size improvement for CTMark.

A simple example of where this can kick in is here:
https://godbolt.org/z/4rra6P

In the example above, GlobalISel currently produces a subs, cset, and tbnz.
SelectionDAG, on the other hand, just emits a compare and b.le.

Differential Revision: https://reviews.llvm.org/D92358