When the timezone is UTC, timestamptz and timestamp are binary coercible
in both directions.  See b8a18ad4 and
c22ecc65 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