[turbofan] Build s/NULL/nullptr/g and CHECK(x != nullptr) to CHECK_NOT_NULL(x).

src/compiler/frame.h

Index: src/compiler/frame.h
diff --git a/src/compiler/frame.h b/src/compiler/frame.h
index 3c9c77aa0af01b8a6ac4921dfd1f20ab3aa19e21..72f756b0dcd8e9693e21b9a5bf88f7ef3132d27f 100644
--- a/src/compiler/frame.h
+++ b/src/compiler/frame.h
@@ -121,12 +121,12 @@ class Frame : public ZoneObject {
   }
 
   void SetAllocatedRegisters(BitVector* regs) {
-    DCHECK(allocated_registers_ == NULL);
+    DCHECK(allocated_registers_ == nullptr);
     allocated_registers_ = regs;
   }
 
   void SetAllocatedDoubleRegisters(BitVector* regs) {
-    DCHECK(allocated_double_registers_ == NULL);
+    DCHECK(allocated_double_registers_ == nullptr);
     allocated_double_registers_ = regs;
   }