[llvm] [AArch64][GlobalISel] Don't crash when legalising G_*MIN/G_*MAX of pointer vector (PR #168872)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > End to end IR tests would be better
> 
> I had planned to use an end to end IR test but unfortunately we still fail to select here so had thought a MIR test would do the job. What would you recommend?

I guess keep it as MIR for now 

https://github.com/llvm/llvm-project/pull/168872