PSR12/OpenTag: improve performance

No functional changes at all, just improving performance of the sniff by changing the order of certain checks.

Some benchmarks for this change run using the Performance report (PR 3810):

Command: `phpcs -ps . --extensions=php --ignore=/vendor/ --report=performance --standard=psr12`

Output for the `PSR12.Files.OpenTag` sniff:

Result | PHPCS itself       | Set of Projects A  | Set of Projects B  | Set of Projects C |
------ | ------------------ | ------------------ | ------------------ | ----------------- |
Nr of Files Scanned | 614   | 4115               | 25546              | 2250              |
Before | 0.077045 (  2.3 %) | 0.982014 (  1.9 %) | 3.267655 (  2.1 %) | 0.179564 (  2.1 %)
After  | 0.000928 (  0.0 %) | 0.013066 (  0.0 %) | 0.091816 (  0.1 %) | 0.109021 (  1.3 %)

With what the sniff does, the impact is biggest for files/codebases which already comply with the expectations of this sniff.

src/Standards/PSR12/Sniffs/Files/OpenTagSniff.php

@@ -44,25 +44,28 @@ public function process(File $phpcsFile, $stackPtr)
             return $phpcsFile->numTokens;
         }
 
-        $next = $phpcsFile->findNext(T_INLINE_HTML, 0);
-        if ($next !== false) {
-            // This rule only applies to PHP-only files.
-            return $phpcsFile->numTokens;
-        }
-
         $tokens = $phpcsFile->getTokens();
         $next   = $phpcsFile->findNext(T_WHITESPACE, ($stackPtr + 1), null, true);
         if ($next === false) {
             // Empty file.
-            return;
+            return $phpcsFile->numTokens;
+        }
+
+        if ($tokens[$next]['line'] !== $tokens[$stackPtr]['line']) {
+            // Tag is on a line by itself.
+            return $phpcsFile->numTokens;
+        }
+
+        $next = $phpcsFile->findNext(T_INLINE_HTML, 0);
+        if ($next !== false) {
+            // This rule only applies to PHP-only files.
+            return $phpcsFile->numTokens;
         }
 
-        if ($tokens[$next]['line'] === $tokens[$stackPtr]['line']) {
-            $error = 'Opening PHP tag must be on a line by itself';
-            $fix   = $phpcsFile->addFixableError($error, $stackPtr, 'NotAlone');
-            if ($fix === true) {
-                $phpcsFile->fixer->addNewline($stackPtr);
-            }
+        $error = 'Opening PHP tag must be on a line by itself';
+        $fix   = $phpcsFile->addFixableError($error, $stackPtr, 'NotAlone');
+        if ($fix === true) {
+            $phpcsFile->fixer->addNewline($stackPtr);
         }
 
         return $phpcsFile->numTokens;