[MachineModuleInfoImpls] Replace qsort with array_pod_sort

Summary:
This seems to be the only place in llvm we directly call qsort. We can replace
this with a call to array_pod_sort. Also minor cleanup of the sorting function.

Reviewers: bkramer, Eugene.Zelenko, rafael

Reviewed By: bkramer

Subscribers: efriedma, llvm-commits

Differential Revision: https://reviews.llvm.org/D39214

llvm-svn: 316671