Optimize ADD-ITERATOR-TO-BAG a few percent

For the case where keys already exist in the Bag.  This same optimization
(using nqp::ifnull(nqp::atkey instead of nqp::if(nqp::existskey...nqp::atkey)
could probably be done for *all* the methods in Rakudo/QuantHash

Spotted by MasterDuke++

src/core.c/Rakudo/QuantHash.pm6

@@ -576,21 +576,30 @@ my class Rakudo::QuantHash {
     }
 
     method ADD-ITERATOR-TO-BAG(\elems, Mu \iterator, Mu \type) {
+        my $pair;
         nqp::until(
           nqp::eqaddr(
             (my \pulled := nqp::decont(iterator.pull-one)),
             IterationEnd
           ),
-          nqp::if(
-            nqp::existskey(elems,(my \which := pulled.WHICH)),
-            nqp::stmts(
-              (my \pair := nqp::atkey(elems,which)),
-              nqp::bindattr(pair,Pair,'$!value',
-                nqp::add_i(nqp::getattr(pair,Pair,'$!value'),1)
-              )
+          nqp::bindattr($pair,Pair,'$!value',nqp::add_i(
+            nqp::getattr(
+              ($pair := nqp::ifnull(
+                nqp::atkey(elems,(my \which := pulled.WHICH)),
+                nqp::if(
+                  nqp::istype(pulled,type),
+                  nqp::bindkey(elems,which,Pair.new(pulled,0)),
+                  X::TypeCheck::Binding.new(
+                    got      => pulled,
+                    expected => type
+                  ).throw
+                )
+              )),
+              Pair,
+              '$!value'
             ),
-            self.BIND-TO-TYPED-BAG(elems, which, pulled, 1, type)
-          )
+            1
+          ))
         );
         elems
     }