fixed behavioural change accidentally introduced by #1348, thanks @satyr

lib/nodes.js

@@ -1702,7 +1702,7 @@
       if (this.isExistentialEquals) {
         return new If(new Existence(fst).invert(), this.second, {
           type: 'if'
-        }).compile(o);
+        }).addElse(fst).compile(o);
       } else {
         return new If(new Existence(fst), ref, {
           type: 'if'

src/nodes.coffee

@@ -1317,7 +1317,7 @@ exports.Op = class Op extends Base
       fst = @first
       ref = fst
     if @isExistentialEquals
-      new If(new Existence(fst).invert(), @second, type: 'if').compile o
+      new If(new Existence(fst).invert(), @second, type: 'if').addElse(fst).compile o
     else
       new If(new Existence(fst), ref, type: 'if').addElse(@second).compile o
 

test/assignment.coffee

@@ -268,3 +268,9 @@ test "existential assignment", ->
   eq nonce, c
   d ?= nonce
   eq nonce, d
+
+test "#1348, #1216: existential assignment comilation", ->
+  nonce = {}
+  a = nonce
+  b = (a ?= 0)
+  eq nonce, b