[Pipeliner] Use std::stable_sort when ordering NodeSets

[Pipeliner] Use std::stable_sort when ordering NodeSets

There are cases when individual NodeSets can be equal with respect to
the ordering criteria. Since they are stored in an ordered container,
use stable_sort to preserve the relative order of equal NodeSets.

This should remove non-determinism discovered by shuffling done in
llvm::sort with expensive checks enabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329915 91177308-0d34-0410-b5e6-96231b3b80d8