regex: avoid using literal optimizations when whitespace is detected

If a literal is entirely whitespace, then it's quite likely that it is
very common. So when that case occurs, just don't do (inner) literal
optimizations at all.

The regex engine may still make sub-optimal decisions here, but that's a
problem for another day.

Fixes #1087

CHANGELOG.md

@@ -14,6 +14,8 @@ Deprecations:
 
 Performance improvements:
 
+* [PERF #1087](https://github.com/BurntSushi/ripgrep/pull/1087):
+  ripgrep is smarter when detected literals are whitespace.
 * [PERF #1381](https://github.com/BurntSushi/ripgrep/pull/1381):
   Directory traversal is sped up with speculative ignore-file existence checks.
 * [PERF cd8ec38a](https://github.com/BurntSushi/ripgrep/commit/cd8ec38a):