[AArch64][GlobalISel] Emit TBZ for SGT cond branches against -1

When we have a G_BRCOND fed by a sgt compare against -1, we can just emit a TBZ.

This is similar to the code in `AArch64TargetLowering::LowerBR_CC`.

Also while we're here, properly scope the commutative constant check in
`selectCompareBranch`, since it sometimes would call
`getConstantVRegValWithLookThrough` twice.

Differential Revision: https://reviews.llvm.org/D74149