WebRequest: Optimise WebRequest::getPathInfo()

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