lval subs: do arg shifting in pp_return

When an lvalue sub does an explicit return, currently pp_return
doesn't touch the args stack and instead tail calls S_return_lvalues()
which does both the leavesuby stuff (e.g. mortalise args) and the returny
stuff (e.g. shift the args down in list context).

Move the call to S_return_lvalues() further down in pp_return so that
the arg shifty stuff is done in pp_return now (like it is for non-lvalue
returns). This will allow us shortly to simply S_return_lvalues.

pp_ctl.c

@@ -2451,9 +2451,6 @@ PP(pp_return)
     }
 
     if (CxTYPE(cx) == CXt_SUB) {
-        if (CvLVALUE(cx->blk_sub.cv))
-            return S_return_lvalues(aTHX_ MARK);
-        else {
             SV **oldsp = PL_stack_base + cx->blk_oldsp;
             if (oldsp != MARK) {
                 /* Handle extra junk on the stack. For example,
@@ -2479,8 +2476,9 @@ PP(pp_return)
                     PL_stack_sp  = oldsp;
             }
             /* fall through to a normal sub exit */
-            return Perl_pp_leavesub(aTHX);
-        }
+            return CvLVALUE(cx->blk_sub.cv)
+                ? S_return_lvalues(aTHX_ NULL)
+                : Perl_pp_leavesub(aTHX);
     }
 
     POPBLOCK(cx,newpm);