tree-optimization/116973 - SLP permute lower heuristic and single-lane SLP

tree-optimization/116973 - SLP permute lower heuristic and single-lane SLP

When forcing single-lane SLP to emulate non-SLP behavior we need to
disable heuristics designed to optimize SLP loads and instead in
all cases resort to an interleaving scheme as requested by forcefully
doing single-lane SLP.

This fixes the remaining fallout for --param vect-force-slp=1 on x86.

	PR tree-optimization/116973
	* tree-vect-slp.cc (vect_lower_load_permutations): Add
	force_single_lane parameter.  Disable heuristic that keeps
	some load-permutations.
	(vect_analyze_slp): Pass force_single_lane to
	vect_lower_load_permutations.