reduce miri code repetition like (n << amt) >> amt

Author: kenta7777

src/librustc_mir/hair/constant.rs

@@ -21,8 +21,7 @@ crate fn lit_to_const<'a, 'gcx, 'tcx>(
         let param_ty = ParamEnv::reveal_all().and(tcx.lift_to_global(&ty).unwrap());
         let width = tcx.layout_of(param_ty).map_err(|_| LitToConstError::Reported)?.size;
         trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
-        let shift = 128 - width.bits();
-        let result = (n << shift) >> shift;
+        let result = truncate(n, width);
         trace!("trunc result: {}", result);
         Ok(ConstValue::Scalar(Scalar::Bits {
             bits: result,