Use return value of XLogRecGetBlockTag instead of explicit block ref checks and also use XLogRecHasBlockRef/Image macros instead of explicit checks

Hi,

I think we can use the return value of XLogRecGetBlockTag instead of
an explicit check XLogRecHasBlockRef as the XLogRecGetBlockTag would
anyways return false in such a case. Also, in some places the macros
XLogRecHasBlockRef and XLogRecHasBlockImage aren't being used instead
the checks are being performed directly.

Although the above change doesn't add any value or improve any
performance but makes use of the existing code better and removes some
unnecessary/duplicate code. Attaching a small patch.

Thoughts?

Regards,
Bharath Rupireddy.