slp: Support optimizing load distribution

This introduces a post processing step for the pattern matcher to flatten
permutes introduced by the complex multiplications patterns.

This performs a blend early such that SLP is not cancelled by the LOAD_LANES
permute.  This is a temporary workaround to the fact that loads are not CSEd
during building and is required to produce efficient code.

gcc/ChangeLog:

	* tree-vect-slp.c (optimize_load_redistribution_1): New.
	(optimize_load_redistribution, vect_is_slp_load_node): New.
	(vect_match_slp_patterns): Use it.