Update test StackCompiler.hs to make it work with GHC > 9.12

Thanks to @AlecsFerra for the fix.

Author: facundominguez

tests/ple/pos/StackCompiler.hs

@@ -11,10 +11,14 @@ module StackCompiler where
 import Prelude hiding ((.))
 import Language.Haskell.Liquid.ProofCombinators
 
-{-@ reflect id @-}
+{-@ assume reflect id as myid @-}
 {-@ reflect $  @-}
 {-@ infix $    @-}
 
+{-@ reflect myid @-}
+myid :: a -> a
+myid x = x
+
 {-@ reflect .  @-}
 {-@ infix .    @-}
 infixr 9 .