Sub-word arithmetic right shift with AArch64 NCG

Summary

An expression involving sub-word arithmetic right shift may yield a wrong value with AArch64 NCG.

This was found while investigating test-primops#8.

Steps to reproduce

{-# LANGUAGE MagicHash #-}
{-# LANGUAGE ExtendedLiterals #-}
import GHC.Word
import GHC.Exts

f :: Int16# -> Word16#
f x = let !w = int16ToWord16# (x `uncheckedShiftRAInt16#` 1#)
      in w `remWord16#` 13#Word16
{-# NOINLINE f #-}

main :: IO ()
main = print (W16# (f (-1798#Int16)))

-- int16ToWord16 (-1798 `shiftR` 1) `rem` 13
--   = int16ToWord16 (-899) `rem` 13
--   = 64637 `rem` 13
--   = 1

$ uname -m
arm64
$ ghc -fforce-recomp subwordarshift.hs
$ ./subwordarshift  # wrong
7
$ ghc -fforce-recomp -fllvm subwordarshift.hs
$ ./subwordarshift  # correct
1

Expected behavior

The reproducer should print 1.

Environment

• GHC version used: 9.8.4, 9.10.2, 9.12.2, 9.13.20250520 (c9abb87c)
• Operating System: macOS
• System Architecture: AArch64