`optimizeExtUses` rewrites uses of an extension's source in other blocks
to read `trunc` of the extension, which is only correct where the
extension isn't `poison`. A `zext nneg` can be valid only where it was
originally used, like behind a check that its source isn't negative. If
that check is one of the rewritten uses, it ends up reading `poison`,
and on `x86_64-pc-windows-msvc`, InstSimplify in WinEHPrepare's cleanup
folds it to true and causes a miscompile.
Drop the extension's poison-generating flags once a use has been
rewritten, like #72103 and #90382 did for similar cases in
`CodeGenPrepare`.
Fixes#234098.
(cherry picked from commit 93fccabb4ebc82695475eaaa279ea5fbb2265450)
dyslexicsteak
395ba2c4d2