Upgrade/Install: Improve `wp_opcache_invalidate()` performance.

Changes from using `preg_match()` based `.php` extension checking to using `substr()` in `wp_opcache_invalidate()`.

Props jnylen0, aaroncampbell, SergeyBiryukov, desrosj, mikeschroder.
Fixes #50784.
Built from https://develop.svn.wordpress.org/trunk@48657


git-svn-id: https://core.svn.wordpress.org/trunk@48419 1a063a9b-81f0-0310-95a4-ce76da25c4cd