Fix go/impossible-interface-nil-check for separate post-update nodes

When tracing back from nil checks on interfaces, ignore post-update
nodes. There will always be a corresponding pre-update node that
contains the information we want.

Author: owen-mc

go/ql/src/RedundantCode/ImpossibleInterfaceNilCheck.ql

@@ -35,7 +35,9 @@ predicate flowsToInterfaceNilCheck(DataFlow::Node nd) {
  */
 predicate nonNilWrapper(DataFlow::Node nd) {
   flowsToInterfaceNilCheck(nd) and
-  forex(DataFlow::Node pred | pred = nd.getAPredecessor() |
+  forex(DataFlow::Node pred |
+    pred = nd.getAPredecessor() and not pred instanceof DataFlow::PostUpdateNode
+  |
     exists(Type predtp | predtp = pred.getType().getUnderlyingType() |
       not predtp instanceof InterfaceType and
       not predtp instanceof NilLiteralType and