[turbofan] Split before loops.

If the range doesn't have calls, but still fails to allocate, try and find a split
position outside a loop.

BUG=

Review URL: https://codereview.chromium.org/1301393012

Cr-Commit-Position: refs/heads/master@{#30580}

src/compiler/greedy-allocator.cc

@@ -476,9 +476,25 @@ LifetimePosition GreedyAllocator::FindSplitPositionAfterCall(
 }
 
 
+LifetimePosition GreedyAllocator::FindSplitPositionBeforeLoops(
+    LiveRange* range) {
+  LifetimePosition end = range->End();
+  if (end.ToInstructionIndex() >= code()->LastInstructionIndex()) {
+    end =
+        LifetimePosition::GapFromInstructionIndex(end.ToInstructionIndex() - 1);
+  }
+  LifetimePosition pos = FindOptimalSplitPos(range->Start(), end);
+  pos = GetSplitPositionForInstruction(range, pos.ToInstructionIndex());
+  return pos;
+}
+
+
 void GreedyAllocator::SplitOrSpillBlockedRange(LiveRange* range) {
   if (TrySplitAroundCalls(range)) return;
-  auto pos = GetLastResortSplitPosition(range, code());
+
+  LifetimePosition pos = FindSplitPositionBeforeLoops(range);
+
+  if (!pos.IsValid()) pos = GetLastResortSplitPosition(range, code());
   if (pos.IsValid()) {
     LiveRange* tail = Split(range, data(), pos);
     DCHECK(tail != range);

src/compiler/greedy-allocator.h

@@ -132,6 +132,9 @@ class GreedyAllocator final : public RegisterAllocator {
   // were made, or false if no calls were found.
   bool TrySplitAroundCalls(LiveRange* range);
 
+  // Find a split position at the outmost loop.
+  LifetimePosition FindSplitPositionBeforeLoops(LiveRange* range);
+
   // Finds the first call instruction in the path of this range. Splits before
   // and requeues that segment (if any), spills the section over the call, and
   // returns the section after the call. The return is: