fix a couple of stylistic clippy warnings

namely:

clippy::redundant_pattern_matching
clippy::redundant_pattern
clippy::search_is_some
clippy::filter_next
clippy::into_iter_on_ref
clippy::clone_on_copy
clippy::needless_return

compiler/rustc_mir/src/borrow_check/diagnostics/region_errors.rs

@@ -387,7 +387,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         if let ReturnConstraint::ClosureUpvar(upvar) = kind {
             let def_id = match self.regioncx.universal_regions().defining_ty {
                 DefiningTy::Closure(def_id, _) => def_id,
-                ty @ _ => bug!("unexpected DefiningTy {:?}", ty),
+                ty => bug!("unexpected DefiningTy {:?}", ty),
             };
 
             let upvar_def_span = self.infcx.tcx.hir().span(upvar);

compiler/rustc_mir/src/transform/instcombine.rs

@@ -126,7 +126,7 @@ impl OptimizationFinder<'b, 'tcx> {
             }
         }
 
-        return None;
+        None
     }
 }