[AArch64][GlobalISel] Emit TBNZ with G_BRCOND where the condition is SLT

When we have a G_ICMP which checks SLT, and the comparison is against 0, we
can emit a TBNZ instead of a CBZ.

This lets us fold in things into the branch, which can provide some code size
savings.

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

https://reviews.llvm.org/D74090