Avoid some table rewrites for ALTER TABLE .. SET DATA TYPE timestamp.

 Avoid some table rewrites for ALTER TABLE .. SET DATA TYPE timestamp.
 
 When the timezone is UTC, timestamptz and timestamp are binary coercible
 in both directions.  See b8a18ad4850ea5ad7884aa6ab731fd392e73b4ad and
 c22ecc6562aac895f0f0529707d7bdb460fd2a49 for the previous attempt in
 this problem space.  Skip the table rewrite; for now, continue to
 needlessly rewrite any index on an affected column.
 
 Reviewed by Simon Riggs and Tom Lane.
 
 Discussion: https://postgr.es/m/20190226061450.GA1665944@rfd.leadboat.com