[CodeGen] Preserve LiveStack analysis in StackSlotColoring pass

For some target architectures, stack slot coloring pass may be invoked
multiple times. It can occur due to the split of RA pass, opening up
the opportunity to optimize stack slots usage at each RA invocation.

In order to achieve that if we could keep stack analysis alive
uptil the invocation of the RA pass for the last time, it will save
up the overhead of recomputing live stack analysis after each RA.
This requires it to be preserved at stack slot coloring pass which
basically optimizes stack slots, but not makes the needed updates
in live stack results. This has been achieved here.

Author: vg0204

llvm/lib/CodeGen/StackSlotColoring.cpp

@@ -149,6 +149,7 @@ namespace {
       AU.addRequired<SlotIndexes>();
       AU.addPreserved<SlotIndexes>();
       AU.addRequired<LiveStacks>();
+      AU.addPreserved<LiveStacks>();
       AU.addRequired<MachineBlockFrequencyInfo>();
       AU.addPreserved<MachineBlockFrequencyInfo>();
       AU.addPreservedID(MachineDominatorsID);
@@ -411,6 +412,23 @@ bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
     }
   }
 
+  // In order to preserve LiveStack analysis, the live ranges for dead spill
+  // stack slots would be merged with the live range of those stack slots that
+  // now share the spill object of the mentioned dead stack slot.
+  for (unsigned SS = 0, SE = SlotMapping.size(); SS != SE; ++SS) {
+    int NewFI = SlotMapping[SS];
+    if (SlotMapping[SS] == -1 || (NewFI == (int)SS)) {
+      continue;
+    }
+
+    LiveRange &lrToUpdateInto =
+        static_cast<LiveRange &>(LS->getInterval(NewFI));
+    const LiveRange &lrToUpdateFrom =
+        static_cast<LiveRange &>(LS->getInterval((int)SS));
+    lrToUpdateInto.MergeSegmentsInAsValue(lrToUpdateFrom,
+                                          lrToUpdateInto.getValNumInfo(0));
+  }
+
   return true;
 }