Use yet another allocator for LiveRanges

Not sure it's worth it for these, there should never be all that
many. We could pre-allocate the maximum size up front.

Author: arsenm

llvm/include/llvm/CodeGen/LiveIntervals.h

@@ -65,12 +65,15 @@ class LiveIntervals {
   MachineDominatorTree *DomTree = nullptr;
   std::unique_ptr<LiveIntervalCalc> LICalc;
 
-  // Allocator for RegUnitRanges and SubRanges.
+  // Allocator for SubRanges.
   BumpPtrAllocator Allocator;
 
   // Allocator for VirtRegIntervals
   SpecificBumpPtrAllocator<LiveInterval> LIAllocator;
 
+  // Allocator for RegUnitRanges
+  SpecificBumpPtrAllocator<LiveRange> LRAllocator;
+
   /// Special pool allocator for VNInfo's (LiveInterval val#).
   VNInfo::Allocator VNInfoAllocator;
 
@@ -423,8 +426,8 @@ class LiveIntervals {
     if (!LR) {
       // Compute missing ranges on demand.
       // Use segment set to speed-up initial computation of the live range.
-      RegUnitRanges[Unit] = LR = new (Allocator.Allocate<LiveRange>())
-          LiveRange(UseSegmentSetForPhysRegs);
+      RegUnitRanges[Unit] = LR =
+          new (LRAllocator.Allocate()) LiveRange(UseSegmentSetForPhysRegs);
       computeRegUnitRange(*LR, Unit);
     }
     return *LR;

llvm/lib/CodeGen/LiveIntervals.cpp

@@ -146,8 +146,7 @@ void LiveIntervals::clear() {
   RegMaskBits.clear();
   RegMaskBlocks.clear();
 
-  for (LiveRange *LR : RegUnitRanges)
-    Allocator.Deallocate(LR);
+  LRAllocator.DestroyAll();
   RegUnitRanges.clear();
 
   // Free the live intervals themselves.