clipboard: cleanup valid_yank_reg and use it for :redir

This fixes missing clipboard support for `:redir`

Helped-By: Michael Reed <m.reed@mykolab.com>
Helped-By: Scott Prager <splinterofchaos@gmail.com>

src/nvim/ex_docmd.c

@@ -6738,8 +6738,7 @@ static void ex_redir(exarg_T *eap)
       /* redirect to a register a-z (resp. A-Z for appending) */
       close_redir();
       ++arg;
-      if (ASCII_ISALPHA(*arg)
-          || *arg == '"') {
+      if (valid_yank_reg(*arg, true) && *arg != '_') {
         redir_reg = *arg++;
         if (*arg == '>' && arg[1] == '>')          /* append */
           arg += 2;