refactor valtree type calculation code

Author: xacrimon

compiler/rustc_mir_build/src/builder/matches/match_pair.rs

@@ -157,23 +157,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         elem_ty: Ty<'tcx>,
     ) -> MatchPairTree<'pat, 'tcx> {
         let tcx = self.tcx;
-        let (const_ty, pat_ty) = match src_pat_ty.kind() {
-            ty::Slice(_) => (
-                Ty::new_imm_ref(
-                    tcx,
-                    tcx.lifetimes.re_erased,
-                    Ty::new_array(tcx, elem_ty, subslice_len),
-                ),
-                Ty::new_slice(tcx, elem_ty),
-            ),
-            ty::Array(_, _) => (
-                Ty::new_imm_ref(
-                    tcx,
-                    tcx.lifetimes.re_erased,
-                    Ty::new_array(tcx, elem_ty, subslice_len),
-                ),
-                Ty::new_array(tcx, elem_ty, subslice_len),
-            ),
+        let const_ty = Ty::new_imm_ref(
+            tcx,
+            tcx.lifetimes.re_erased,
+            Ty::new_array(tcx, elem_ty, subslice_len),
+        );
+
+        let pat_ty = match src_pat_ty.kind() {
+            ty::Slice(_) => Ty::new_slice(tcx, elem_ty),
+            ty::Array(_, _) => Ty::new_array(tcx, elem_ty, subslice_len),
             _ => unreachable!(),
         };