[turbofan] Initial version of allocation folding and write barrier elimination.

src/compiler/linkage.h

Index: src/compiler/linkage.h
diff --git a/src/compiler/linkage.h b/src/compiler/linkage.h
index bf299d39a6990c5b5586d0f321c28bcfe330511d..958e8dc94989736987cbb34a08035fc07377c3bc 100644
--- a/src/compiler/linkage.h
+++ b/src/compiler/linkage.h
@@ -162,7 +162,9 @@ class CallDescriptor final : public ZoneObject {
     kRestoreJSSP = 1u << 6,
     kRestoreCSP = 1u << 7,
     // Causes the code generator to initialize the root register.
-    kInitializeRootRegister = 1u << 8
+    kInitializeRootRegister = 1u << 8,
+    // Does not ever try to allocate space on our heap.
+    kNoAllocate = 1u << 9
   };
   typedef base::Flags<Flag> Flags;