Auto-compute live intervals on demand.

When new virtual registers are created during splitting/spilling, defer
creation of the live interval until we need to use the live interval.

Along with the recent commits to notify LiveRangeEdit when new virtual
registers are created, this makes it possible for functions like
TargetInstrInfo::loadRegFromStackSlot() and
TargetInstrInfo::storeRegToStackSlot() to create multiple virtual
registers as part of the process of generating loads/stores for
different register classes, and then have the live intervals for those
new registers computed when they are needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188437 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SplitKit.cpp

@@ -463,11 +463,11 @@ VNInfo *SplitEditor::defFromParent(unsigned RegIdx,
 unsigned SplitEditor::openIntv() {
   // Create the complement as index 0.
   if (Edit->empty())
-    Edit->create();
+    Edit->createEmptyInterval();
 
   // Create the open interval.
   OpenIdx = Edit->size();
-  Edit->create();
+  Edit->createEmptyInterval();
   return OpenIdx;
 }
 
@@ -1116,7 +1116,7 @@ void SplitEditor::finish(SmallVectorImpl<unsigned> *LRMap) {
     SmallVector<LiveInterval*, 8> dups;
     dups.push_back(li);
     for (unsigned j = 1; j != NumComp; ++j)
-      dups.push_back(&Edit->create());
+      dups.push_back(&Edit->createEmptyInterval());
     ConEQ.Distribute(&dups[0], MRI);
     // The new intervals all map back to i.
     if (LRMap)