Rollup merge of #72425 - RalfJung:discr-sign-ext, r=nikomatsakis

fix discriminant_value sign extension

Fixes a regression introduced in https://github.com/rust-lang/rust/pull/70705
r? @nikomatsakis Cc @lcnr @oli-obk

src/librustc_mir/interpret/intrinsics.rs

@@ -219,7 +219,10 @@ pub fn emulate_intrinsic(
                 let place = self.deref_operand(args[0])?;
                 let discr_val = self.read_discriminant(place.into())?.0;
                 let scalar = match dest.layout.ty.kind {
-                    ty::Int(_) => Scalar::from_int(discr_val as i128, dest.layout.size),
+                    ty::Int(_) => Scalar::from_int(
+                        self.sign_extend(discr_val, dest.layout) as i128,
+                        dest.layout.size,
+                    ),
                     ty::Uint(_) => Scalar::from_uint(discr_val, dest.layout.size),
                     _ => bug!("invalid `discriminant_value` return layout: {:?}", dest.layout),
                 };