WebRequest: Optimise WebRequest::getPathInfo()
Called for all PHP web requests from WebStart.php via
WebRequest::interpolateTitle.
* Use isset() instead of empty() where we only need to check
that the key is supported.
* Don't import global `$wgUsePathInfo` in the common case.
* Migrate from deprecated wrapper Wikimedia\suppressWarnings
to AtEase::suppressWarnings.
* Use strpos() instead of preg_match(). Consistently faster,
albeit not by much (for 100 iterations: 0.04ms vs 0.23ms).
* Don't create unused $matches array for the common case.
Bug: T189966
Change-Id: I0de126953c25f3629cb85a0d4e46598baf261c15