[turbofan] Solidify OSR deconstruction code a bit.

src/compiler/osr.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/osr.h"
 #include "src/ast/scopes.h"
 #include "src/compilation-info.h"
 #include "src/compiler/all-nodes.h"
 #include "src/compiler/common-operator-reducer.h"
 #include "src/compiler/common-operator.h"
@@ skipping 253 matching lines @@
   Node* osr_loop = nullptr;
 
   for (Node* node : graph->start()->uses()) {
     if (node->opcode() == IrOpcode::kOsrLoopEntry) {
       osr_loop_entry = node;  // found the OSR loop entry
     } else if (node->opcode() == IrOpcode::kOsrNormalEntry) {
       osr_normal_entry = node;
     }
   }
 
-  if (osr_loop_entry == nullptr) {
-    // No OSR entry found, do nothing.
-    CHECK(osr_normal_entry);
-    return;
-  }
+  CHECK_NOT_NULL(osr_normal_entry);  // Should have found the OSR normal entry.
+  CHECK_NOT_NULL(osr_loop_entry);    // Should have found the OSR loop entry.
 
   for (Node* use : osr_loop_entry->uses()) {
     if (use->opcode() == IrOpcode::kLoop) {
       CHECK(!osr_loop);             // should be only one OSR loop.
       osr_loop = use;               // found the OSR loop.
     }
   }
 
   CHECK(osr_loop);  // Should have found the OSR loop.
 
@@ skipping 46 matching lines @@
 
 void OsrHelper::SetupFrame(Frame* frame) {
   // The optimized frame will subsume the unoptimized frame. Do so by reserving
   // the first spill slots.
   frame->ReserveSpillSlots(UnoptimizedFrameSlots());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8