[turbofan] Validation for deferred->hot transition edges

Validate that the transition from deferred to hot happens through a
deferred block with one successor. This is needed for frame elision: if
we need to deconstruct the frame on the deferred path, this extra block
offers that location.

A precondition for this validation is that the sequence is in split edge
form.

BUG=

Review URL: https://codereview.chromium.org/1811213003

Cr-Commit-Position: refs/heads/master@{#35015}

src/compiler/instruction.cc

@@ -635,6 +635,17 @@ void InstructionSequence::ValidateEdgeSplitForm() {
   }
 }
 
+void InstructionSequence::ValidateDeferredBlockExitPaths() {
+  // A deferred block with more than one successor must have all its successors
+  // deferred.
+  for (const InstructionBlock* block : instruction_blocks()) {
+    if (!block->IsDeferred() || block->SuccessorCount() <= 1) continue;
+    for (RpoNumber successor_id : block->successors()) {
+      CHECK(InstructionBlockAt(successor_id)->IsDeferred());
+    }
+  }
+}
+
 void InstructionSequence::ValidateSSA() {
   // TODO(mtrofin): We could use a local zone here instead.
   BitVector definitions(VirtualRegisterCount(), zone());

src/compiler/instruction.h

@@ -1355,6 +1355,7 @@ class InstructionSequence final : public ZoneObject {
   void PrintBlock(int block_id) const;
 
   void ValidateEdgeSplitForm();
+  void ValidateDeferredBlockExitPaths();
   void ValidateSSA();
 
  private:

src/compiler/pipeline.cc

@@ -1446,6 +1446,7 @@ void Pipeline::AllocateRegisters(const RegisterConfiguration* config,
 #ifdef DEBUG
   debug_name = info()->GetDebugName();
   data_->sequence()->ValidateEdgeSplitForm();
+  data_->sequence()->ValidateDeferredBlockExitPaths();
 #endif
 
   data->InitializeRegisterAllocationData(config, descriptor, debug_name.get());