QLibrary: succeed early for .dylib on Darwin

Drag the cheap check for 'fileName.endsWith(".dylib")' to before the
expensive operations (QFileInfo creation, extraction and splitting of
completeSuffix(), building of suffix candidate list, ...), at the cost
of a duplicated Q_OS_DARWIN check.

Change-Id: I8ed764f18f0beb8ad24c30ab806ecc5452159601
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/plugin/qlibrary.cpp

@@ -664,6 +664,11 @@ bool QLibrary::isLibrary(const QString &fileName)
 #if defined(Q_OS_WIN)
     return fileName.endsWith(QLatin1String(".dll"), Qt::CaseInsensitive);
 #else // Generic Unix
+# if defined(Q_OS_DARWIN)
+    // On Apple platforms, dylib look like libmylib.1.0.0.dylib
+    if (fileName.endsWith(QLatin1String(".dylib")))
+        return true;
+# endif
     QString completeSuffix = QFileInfo(fileName).completeSuffix();
     if (completeSuffix.isEmpty())
         return false;
@@ -683,10 +688,6 @@ bool QLibrary::isLibrary(const QString &fileName)
 # elif defined(Q_OS_AIX)
     validSuffixList << QLatin1String("a") << QLatin1String("so");
 # elif defined(Q_OS_DARWIN)
-    // On Apple platforms, dylib look like libmylib.1.0.0.dylib
-    if (suffixes.last() == QLatin1String("dylib"))
-        return true;
-
     validSuffixList << QLatin1String("so") << QLatin1String("bundle");
 # elif defined(Q_OS_UNIX)
     validSuffixList << QLatin1String("so");