Rationalize expression context reset in ExecModifyTable().

Rationalize expression context reset in ExecModifyTable().

The current pattern of reseting expressions both in
ExecProcessReturning() and ExecOnConflictUpdate() makes it harder than
necessary to reason about memory lifetimes.  It also requires
materializing slots unnecessarily, although this patch doesn't take
advantage of the fact that that's not necessary anymore.

Instead reset the expression context once for each input tuple.

Author: Ashutosh Bapat
Discussion: https://postgr.es/m/20181105210039.hh4vvi4vwoq5ba2q@alap3.anarazel.de