[GlobalMerge] Stable sort GlobalSets to fix non-deterministic sort order

Summary: This fixes failure in CodeGen/AArch64/global-merge-group-by-use.ll uncovered by D39245.

Reviewers: ab, asl

Reviewed By: ab

Subscribers: aemerson, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D39635

llvm-svn: 317817