Revert "unique_ptrify LiveRange::segmentSet"

Apparently something does care about ordering of LiveIntervals... so
revert all that stuff (r231175, r231176, r231177) & take some time to
re-evaluate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231184 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/LiveInterval.cpp

@@ -743,6 +743,7 @@ void LiveRange::flushSegmentSet() {
       segments.empty() &&
       "segment set can be used only initially before switching to the array");
   segments.append(segmentSet->begin(), segmentSet->end());
+  delete segmentSet;
   segmentSet = nullptr;
   verify();
 }