Revert of [turbofan] simplify gap ordering

src/compiler/register-allocator.cc

Index: src/compiler/register-allocator.cc
diff --git a/src/compiler/register-allocator.cc b/src/compiler/register-allocator.cc
index 416b970865f6bd95d9bdfee0bb90c8b2775bb320..9eb4a470f298359c1ae5cbb14d9cf314bf13d737 100644
--- a/src/compiler/register-allocator.cc
+++ b/src/compiler/register-allocator.cc
@@ -1078,7 +1078,7 @@
       for (auto succ : block->successors()) {
         const InstructionBlock* successor = code()->InstructionBlockAt(succ);
         DCHECK(successor->PredecessorCount() == 1);
-        int gap_index = successor->first_instruction_index();
+        int gap_index = successor->first_instruction_index() + 1;
         DCHECK(code()->IsGapAt(gap_index));
 
         // Create an unconstrained operand for the same virtual register
@@ -1095,7 +1095,7 @@
       for (auto succ : block->successors()) {
         const InstructionBlock* successor = code()->InstructionBlockAt(succ);
         DCHECK(successor->PredecessorCount() == 1);
-        int gap_index = successor->first_instruction_index();
+        int gap_index = successor->first_instruction_index() + 1;
         range->SpillAtDefinition(local_zone(), gap_index, output);
         range->SetSpillStartIndex(gap_index);
       }
@@ -2376,8 +2376,7 @@
 
 bool RegisterAllocator::IsBlockBoundary(LifetimePosition pos) {
   return pos.IsInstructionStart() &&
-         code()->GetInstructionBlock(pos.InstructionIndex())->code_start() ==
-             pos.InstructionIndex();
+         InstructionAt(pos.InstructionIndex())->IsBlockStart();
 }
 
 
@@ -2475,13 +2474,9 @@
     // The split result intersects with [start, end[.
     // Split it at position between ]start+1, end[, spill the middle part
     // and put the rest to unhandled.
-    auto third_part_end = end.PrevInstruction().InstructionEnd();
-    if (IsBlockBoundary(end.InstructionStart())) {
-      third_part_end = end.InstructionStart();
-    }
     auto third_part = SplitBetween(
         second_part, Max(second_part->Start().InstructionEnd(), until),
-        third_part_end);
+        end.PrevInstruction().InstructionEnd());
     if (!AllocationOk()) return;
 
     DCHECK(third_part != second_part);