Add NullablePtr type

src/ir/effects.h

diff --git a/src/ir/effects.h b/src/ir/effects.h
index 4d4d5d5..b6943c5 100644
--- a/src/ir/effects.h
+++ b/src/ir/effects.h
@@ -1330,7 +1330,7 @@
     template<typename CallType>
     void
     addCallEffects(const CallType* curr,
-                   const EffectAnalyzer* bodyEffects) {
+                   NullablePtr<const EffectAnalyzer*> bodyEffects) {
       if (curr->isReturn) {
         parent.branchesOut = true;
       }

src/support/utilities.h

diff --git a/src/support/utilities.h b/src/support/utilities.h
index 272488e..8240409 100644
--- a/src/support/utilities.h
+++ b/src/support/utilities.h
@@ -105,6 +105,11 @@
   using Ts::operator()...;
 };
 
+// Used to annotate (for human readers) that a pointer may be null.
+// e.g. NullablePtr<int*>
+template<typename T>
+using NullablePtr = T;
+
 } // namespace wasm
 
 #endif // wasm_support_utilities_h