docs: kdoc: micro-optimize KernRe

Switch KernRe::add_regex() to a try..except block to avoid looking up each
regex twice.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/lib/kdoc/kdoc_re.py

@@ -29,12 +29,10 @@ class KernRe:
         """
         Adds a new regex or re-use it from the cache.
         """
-
-        if string in re_cache:
+        try:
             self.regex = re_cache[string]
-        else:
+        except KeyError:
             self.regex = re.compile(string, flags=flags)
-
             if self.cache:
                 re_cache[string] = self.regex