Simplify check of modified attributes in heap_update

Simplify check of modified attributes in heap_update

The old coding was getting more complicated as new things were added,
and it would be barely tolerable with upcoming WARM updates and other
future features such as indirect indexes.  The new coding incurs a small
performance cost in synthetic benchmark cases, and is barely measurable
in normal cases.  A much larger benefit is expected from WARM, which
could actually bolt its needs on top of the existing coding, but it is
much uglier and bug-prone than doing it on this new code.  Additional
optimization can be applied on top of this, if need be.

Reviewed-by: Pavan Deolasee, Amit Kapila, Mithun CY
Discussion: https://postgr.es/m/20161228232018.4hc66ndrzpz4g4wn@alvherre.pgsql
	https://postgr.es/m/CABOikdMJfz69dBNRTOZcB6s5A0tf8OMCyQVYQyR-WFFdoEwKMQ@mail.gmail.com