Lower discriminant_value intrinsic

This allows const propagation to evaluate comparisons involving
field-less enums using derived implementations of `PartialEq` (after
inlining `eq`).

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -489,6 +489,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
             mir::Rvalue::Discriminant(ref place) => {
                 let discr_ty = rvalue.ty(self.mir, bx.tcx());
+                let discr_ty = self.monomorphize(discr_ty);
                 let discr = self
                     .codegen_place(&mut bx, place.as_ref())
                     .codegen_get_discr(&mut bx, discr_ty);