Turn off optimization for checkForAlphaPixels on Windows ARM

This function is optimized in a way that it returns bad results with the
arm msvc compiler.

Task-number: QTBUG-52007
Change-Id: Ica4517dfb7dde5a78d563f9253c4c907090c9459
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>
Reviewed-by: Kevin Funk <kevin.funk@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/image/qimage.cpp

@@ -170,6 +170,9 @@ QImageData::~QImageData()
     data = 0;
 }
 
+#if defined(_M_ARM)
+#pragma optimize("", off)
+#endif
 
 bool QImageData::checkForAlphaPixels() const
 {
@@ -277,6 +280,9 @@ bool QImageData::checkForAlphaPixels() const
 
     return has_alpha_pixels;
 }
+#if defined(_M_ARM)
+#pragma optimize("", on)
+#endif
 
 /*!
     \class QImage