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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@317817 91177308-0d34-0410-b5e6-96231b3b80d8

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