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

compiler/semstmts.nim

@@ -135,7 +135,7 @@ const
   skipForDiscardable = {nkStmtList, nkStmtListExpr,
     nkOfBranch, nkElse, nkFinally, nkExceptBranch,
     nkElifBranch, nkElifExpr, nkElseExpr, nkBlockStmt, nkBlockExpr,
-    nkHiddenStdConv, nkHiddenDeref}
+    nkHiddenStdConv, nkHiddenSubConv, nkHiddenDeref}
 
 proc implicitlyDiscardable(n: PNode): bool =
   # same traversal as endsInNoReturn

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()