slp: elide intermediate nodes for complex add and avoid truncate

slp: elide intermediate nodes for complex add and avoid truncate

This applies the same feedback received for MUL and the rest to
ADD which was already committed.  In short it elides the intermediate
nodes vec and avoids the use of truncate on the SLP child.

gcc/ChangeLog:

	* tree-vect-slp-patterns.c (complex_add_pattern::build):
	Elide nodes.