fixes semi-regression; discard check now skips nkHiddenSubConv (#23840)

follow up https://github.com/nim-lang/Nim/pull/23681

ref https://forum.nim-lang.org/t/11987

tests/discard/tdiscardable.nim

@@ -164,3 +164,12 @@ block: # bug #23775
       let _ = retString()
 
   doStuff(C)
+
+block:
+  proc test(): (int, int) {.discardable.} =
+    discard
+
+  if true:
+    test()
+  else:
+    quit()